ath9k: Handle TSF properly for AP mode

The TSF has to be reset only once, upon bringing
the interface up in AP mode. For any beacon reconfigure calls
after that, resetting the TSF results in incorrect beacon generation.
The only exception is a change in the beacon interval,
which is indicated to the driver by mac80211 through
IEEE80211_CONF_CHANGE_BEACON_INTERVAL, handle this properly.

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