ath9k: send powersave frame on channel switch

While leaving from or entering to active channel context, send out
nullfunc frame to inform to the AP about the presence of station.

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