ath9k: serialize ath9k_hw_setpower calls

Because ath9k_setpower is called from various contexts, we have to
protect it against concurrent calls.

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