ath5k: update beacons in AP mode

ath5k only generated the beacon when bss_info_changed() was called,
but for AP mode this is not enough, because the TIM IE would never
get updated and consequently PS mode clients wouldn't know about
buffered frames.  Instead, get a new beacon on every SWBA interrupt.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>