ath9k: Optimize TBTT/DTIM calculation for timers

The previous version used a simple loop to go through all Beacon
frames when determining the next TBTT and DTIM count. This is not too
bad for the case where the setup happens before timesync (i.e., very
small TSF), but this can become very heavy operation if a short Beacon
interval is used and the current TSF is large.

In preparation for a patch to update timer setup based on Beacon
timestamp, optimize this routine to take fixed time regardless of the
actual TSF value.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>