mfd: ti_am335x_tscadc: Fix struct clk memory leak

mfd: ti_am335x_tscadc: Fix struct clk memory leak

Use devm_elk_get() to let Linux manage struct clk memory to avoid the following
memory leakage report:

unreferenced object 0xdd75efc0 (size 64):
  comm "systemd-udevd", pid 186, jiffies 4294945126 (age 1195.750s)
  hex dump (first 32 bytes):
    61 64 63 5f 74 73 63 5f 66 63 6b 00 00 00 00 00  adc_tsc_fck.....
    00 00 00 00 92 03 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<c0a15260>] kmemleak_alloc+0x40/0x74
    [<c0287a10>] __kmalloc_track_caller+0x198/0x388
    [<c0255610>] kstrdup+0x40/0x5c
    [<c025565c>] kstrdup_const+0x30/0x3c
    [<c0636630>] __clk_create_clk+0x60/0xac
    [<c0630918>] clk_get_sys+0x74/0x144
    [<c0630cdc>] clk_get+0x5c/0x68
    [<bf0ac540>] ti_tscadc_probe+0x260/0x468 [ti_am335x_tscadc]
    [<c06f3c0c>] platform_drv_probe+0x60/0xac
    [<c06f1abc>] driver_probe_device+0x214/0x2dc
    [<c06f1c18>] __driver_attach+0x94/0xc0
    [<c06efe2c>] bus_for_each_dev+0x90/0xa0
    [<c06f1470>] driver_attach+0x28/0x30
    [<c06f1030>] bus_add_driver+0x184/0x1ec
    [<c06f2b74>] driver_register+0xb0/0xf0
    [<c06f3b4c>] __platform_driver_register+0x40/0x54

Signed-off-by: Zumeng Chen <zumeng.chen@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>