pinctrl: mt7622: fix a kernel panic when pio don't work as EINT controller

pinctrl: mt7622: fix a kernel panic when pio don't work as EINT controller

The function, external interrupt controller, is made as an optional to
mt7622 pinctrl. But if we don't want pio behaves as an external interrupt
controller, it would lead to hw->eint not be created properly and then
will cause 'kernel NULL pointer' issue when gpiochip try to call .to_irq
or .set_config. To fix it, check hw->eint before accessing the member.

[    1.339494] Unable to handle kernel NULL pointer dereference at virtual
	       address 00000010
[    1.347857] Mem abort info:
[    1.350742]   ESR = 0x96000005
[    1.353905]   Exception class = DABT (current EL), IL = 32 bits
[    1.360024]   SET = 0, FnV = 0
[    1.363185]   EA = 0, S1PTW = 0
[    1.366431] Data abort info:
[    1.369405]   ISV = 0, ISS = 0x00000005
[    1.373363]   CM = 0, WnR = 0
[    1.376437] [0000000000000010] user address but active_mm is swapper
[    1.383005] Internal error: Oops: 96000005 [#1] PREEMPT SMP
[    1.388748] Modules linked in:
[    1.391897] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.16.0-rc1+ #344
[    1.398625] Hardware name: MediaTek MT7622 RFB1 board (DT)
[    1.404279] pstate: 80000005 (Nzcv daif -PAN -UAO)
[    1.409221] pc : mtk_eint_find_irq+0x8/0x24
[    1.413532] lr : mtk_gpio_to_irq+0x20/0x28
[    1.417749] sp : ffffff800801baf0
[    1.421161] x29: ffffff800801baf0 x28: ffffff8008792f40
[    1.426637] x27: ffffff800886b000 x26: ffffff8008615620
[    1.432113] x25: ffffffc00e4dbdc8 x24: ffffff80087b8000
[    1.437589] x23: ffffffc00325a000 x22: ffffffc00325a010
[    1.443066] x21: ffffffc0033dec18 x20: 00000000ffffffea
[    1.448542] x19: ffffffc00e4db800 x18: 0000000000000130
[    1.454018] x17: 000000000000000e x16: 0000000000000007
[    1.459494] x15: ffffff80085ee000 x14: 0000000000000001
[    1.464970] x13: 0000000000000001 x12: 0000000000000010
[    1.470446] x11: 0101010101010101 x10: 0000000000000880
[    1.475922] x9 : ffffff800801b990 x8 : ffffffc0030688e0
[    1.481399] x7 : ffffff80080c0660 x6 : ffffffc00e4dbbb0
[    1.486875] x5 : 0000000000000000 x4 : 0000000000000000
[    1.492351] x3 : ffffff80082a92f4 x2 : 00000000fffffffa
[    1.497826] x1 : 0000000000000051 x0 : 0000000000000000
[    1.503305] Process swapper/0 (pid: 1, stack limit = 0x0000000054e053bd)
[    1.510210] Call trace:
[    1.512727]  mtk_eint_find_irq+0x8/0x24
[    1.516677]  mtk_gpio_to_irq+0x20/0x28
[    1.520539]  gpiod_to_irq+0x48/0x60
[    1.524135]  mmc_gpiod_request_cd_irq+0x3c/0xc4
[    1.528804]  mmc_start_host+0x6c/0x8c
[    1.532575]  mmc_add_host+0x58/0x7c
[    1.536168]  msdc_drv_probe+0x4fc/0x67c
[    1.540121]  platform_drv_probe+0x58/0xa4
[    1.544251]  driver_probe_device+0x204/0x44c
[    1.548649]  __driver_attach+0x84/0xf8
[    1.552512]  bus_for_each_dev+0x68/0xa0
[    1.556461]  driver_attach+0x20/0x28
[    1.560142]  bus_add_driver+0xec/0x240
[    1.564002]  driver_register+0x98/0xe4
[    1.567863]  __platform_driver_register+0x48/0x50
[    1.572711]  mt_msdc_driver_init+0x18/0x20
[    1.576932]  do_one_initcall+0x98/0x130
[    1.580886]  kernel_init_freeable+0x13c/0x1d4
[    1.585375]  kernel_init+0x10/0xf8
[    1.588879]  ret_from_fork+0x10/0x18
[    1.592564] Code: a8c67bfd d65f03c0 a9bf7bfd 910003fd (f9400800)
[    1.598849] ---[ end trace 4bbcb7bc30e98492 ]---
[    1.603677] Kernel panic - not syncing: Attempted to kill init!
	       exitcode=0x0000000b
[    1.603677]

cc: Kevin Hilman <khilman@baylibre.com>
Cc: stable@vger.kernel.org
Fixes: e6dabd38d8e7 ("pinctrl: mediatek: add EINT support to MT7622 SoC")
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>