ath9k: serialize ath9k_ps_{wakeup,restore} calls

These functions are changing the power mode of the chip, but this may
have unpredictable effects, if another code are trying to set the power
mode via 'ath9k_hw_setpower' in the same time from another context.

Changes-licensed-under: ISC
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>