ath9k: channel context based transmission

Force queueing of all frames that belong to a virtual interface on
a different channel context, to ensure that they are sent on the
correct channel.

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>