ath9k: Access rchan::buf only with per_cpu helper

ath9k: Access rchan::buf only with per_cpu helper

The relayfs was changed to use per CPU constructs to handle the rchan
buffers. But the users of the rchan buffers in other parts of the kernel
were not modified. This caused crashes like

  BUG: unable to handle kernel paging request at 00003a5198a0b910
  IP: [<ffffffffa973cb3a>] ath_cmn_process_fft+0xea/0x610
  PGD 0 [  179.522449]
  Oops: 0000 [#1] SMP
  Modules linked in:
  CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.9.0-rc5 #1
  [...]
  Call Trace:
   <IRQ> [  179.656426]  [<ffffffffa9704373>] ? ath_rx_tasklet+0x2f3/0xd10
   [<ffffffffa9702106>] ? ath9k_tasklet+0x1b6/0x230
   [<ffffffffa90dcbd1>] ? tasklet_action+0xf1/0x100
   [<ffffffffa9a3cb3f>] ? __do_softirq+0xef/0x284
   [<ffffffffa90dd22e>] ? irq_exit+0xae/0xb0
   [<ffffffffa9a3c89f>] ? do_IRQ+0x4f/0xd0
   [<ffffffffa9a3aa42>] ? common_interrupt+0x82/0x82
   <EOI> [  179.703152]  [<ffffffffa9a39c1d>] ? poll_idle+0x2d/0x57
   [<ffffffffa908c845>] ? sched_clock+0x5/0x10
   [<ffffffffa97bc8d6>] ? cpuidle_enter_state+0xf6/0x2d0
   [<ffffffffa911988e>] ? cpu_startup_entry+0x14e/0x230
   [<ffffffffaa3cdf70>] ? start_kernel+0x461/0x481
   [<ffffffffaa3cd120>] ? early_idt_handler_array+0x120/0x120
   [<ffffffffaa3cd413>] ? x86_64_start_kernel+0x14c/0x170
  Code: 31 db 41 be ff ff ff ff 4c 8b 26 48 8b 6e 08 49 8b 84 24 60 05 00
        00 48 8b 00 0f b7 40 04 66 89 44 24 48 eb 11 48 8b 55 40 48 98 <48>
        8b 3c c2 e8 ad a0 a4 ff 01 c3 41 8d 56 01 be 00 02 00 00 48
  RIP  [<ffffffffa973cb3a>] ath_cmn_process_fft+0xea/0x610
   RSP <ffff9b43e7003d20>
  CR2: 00003a5198a0b910

Fixes: 017c59c042d0 ("relay: Use per CPU constructs for the relay channel buffer pointers")
Cc: Akash Goel <akash.goel@intel.com>
Cc: Nick Kossifidis <mickflemm@gmail.com>
Reported-by: Mathias Kretschmer <mathias.kretschmer@fit.fraunhofer.de>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>