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 64badcb6 ("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>

drivers/net/wireless/ath/ath10k/mac.c

@@ -2839,7 +2839,7 @@ static int ath10k_add_interface(struct ieee80211_hw *hw,
 		arvif->beacon_buf = dma_zalloc_coherent(ar->dev,
 							IEEE80211_MAX_FRAME_LEN,
 							&arvif->beacon_paddr,
-							GFP_KERNEL);
+							GFP_ATOMIC);
 		if (!arvif->beacon_buf) {
 			ret = -ENOMEM;
 			ath10k_warn(ar, "failed to allocate beacon buffer: %d\n",