ath10k: fix kernel panic while shutting down AP

Based on GFP flag given to DMA coherent allocation, the behaviour of
dma_free_coherent is changed. This behavioural diffrence is noticeable
in ARM platform. If DMA memory is allocated with GFP_KERNEL, free
coherent can not be called inside spin lock. This is causing kernel
crash in ARM platforms. Fix this by changing GFP flag to atomic.

This is most likely a regression from commit 64badcb6d645 ("ath10k: workaround
fw beaconing bug").

Cc: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Rajkumar Manoharan <rmanohar@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>