ath9k: Adjust AP beacon tsf based on station context

In multi channel context (AP + STA case), adjust the TSF time of
the AP chanctx to keep its beacons at half beacon interval offset
relative to the STA chanctx.

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>