ath5k: use SWBA to detect IBSS HW merges

use SWBA (software beacon alert) interrupts to keep track of the next beacon
time und check if a HW merge (automatic TSF update) has happened on every
received beacon with the same BSSID.

this is necessary because the atheros hardware will silently update the local
TSF in IBSS mode, but not its beacon timers. if the TSF is ahead of the beacon
timers no beacons are sent until the timers wrap around (typically after about
1 minute).

this solution is not very nice, since we have to look into every beacon, but
there is apparently no other way to detect HW merges.

drivers/net/wireless/ath5k/base.c:      Changes-licensed-under: 3-Clause-BSD
drivers/net/wireless/ath5k/base.h:      Changes-licensed-under: 3-Clause-BSD

Signed-off-by: Bruno Randolf <bruno@thinktube.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>