ath9k: Fix deadlock while updating p2p beacon timer

pm_lock is taken twice while syncing HW TSF of p2p vif.
Fix this by taking the lock at caller side.

Cc: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Rajkumar Manoharan <rmanohar@qti.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>