ath9k: Implement channel context ops

Add channel context operations (add, remove, change, assign and
unassign) to enable support for multiple channels.

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>