ath9k: Use mutex_lock to avoid potential race in start/stop rng

Move ath9k_rng_stop/ath9k_rng_start pair into critical section,
use mutex_lock to void potential race accessing.

Signed-off-by: Miaoqing Pan <miaoqing@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>