ath9k_htc: Fix TBTT calculation for IBSS mode

The target beacon transmission time has to be synced with the HW
TSF when configuring beacon timers in Adhoc mode. Failing to do this
would cause erroneous beacon transmission, for example, on completion
of a scan run to check for IBSS merges.

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