[ath] revert the previous commit, after reading the 802.11-2012 spec a bit more.

The 802.11-2012 spec talks about this - section 10.1.3.2 - Beacon Generation
in Infrastructure Networks. So yes, we should be expecting beacons to be
going out in multiples of intval.

Silly adrian.

So:

• fix the FreeBSD APs that are sending beacons at incorrect TBTTs (target beacon transmit time); and
• yes indeed we will have to wake up out of network sleep until we sync a beacon.