ath9k: Correct TSF adjustment to align the beacon time correctly

Beacons were not send out at (timestamp % beacon_time == 0) for interfaces
other than the primary one. To send out beacons with the correct timestamp
according to 10.1.3.2 of the 802.11 standard the tsf_adjustment has to be
set to the negative time difference instead of positive. This way the
later beacons get corrected to have a lower (and similar) timestamp with
regard to the beacon from slot 0.

I am not aware about any issues that have been caused by this.

Signed-off-by: Benjamin Berg <benjamin.berg@open-mesh.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>