[if_ath] Don't update the beacon bits from beacon frames in hostapd mode.

This logic is running the beacon receive bits in STA+AP mode on both the
STA and AP side. The STA side sees its beacons from the BSS fine; the
AP side is seeing other beacons on the same channel but with the BSS
node for some odd reason. (I think it's a valid reason, but I currently
forget what that valid reason is.)

So, just to be cleaner about things, don't run the nexttbtt/etc bits
at all if we're in hostap mode. If I ever get mesh working then maybe
I'll make sure it works right on mesh+ap and mesh+sta modes.

Whilst here, log the VAP i'm being called on to make it clearer what
is going on. I may end up adding a VAP dprintf version of this at
some point.

Tested:

• AR9380, STA (DWDS client) + hostap on the same NIC