ath9k_htc: Add TSF adjust capability

In multi-interface mode, beacons/probe responses that are
sent out must have their timestamp field updated. Calculate
the TSF adjustment value for each beaconing interface and set it
in the frame properly.

Signed-off-by: Sujith Manoharan <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>