[ath] Don't re-program the beacon timers if we miss a beacon in software-beacon STA mode.

This is something I added a few years ago to handle resyncing the beacon if
we miss a beacon or need to sync after association/reassociation/powersave.

However, if we're doing STA+AP mode (eg DWDS) then we don't want
to reprogram the beacons here; this may upset normal AP operation.
I missed checking for the sc->sc_swbmiss flag so I was reinitialising
the beacon timers after every beacon miss / TSFOOR option, and
that isn't likely good.

This plus ensuring that STA's are created with "-beacon" to disable
BMISS/TSFOOR processing will hopefully quieten some of the issues
I've seen with missed beacons / TSFOOR (out of range) interrupts
coming in when operating in STA mode.

Tested:

• AR9380/AR9580, STA+AP modes