[net80211] revert part of r282405 in order to restore IBSS behaviour.

This prevents nodes being created for peers on BSSes that are not our own.
(Ie, same channel, IBSS, but different BSS.)

The "IBSS merge" thing was fixed by me enabling "see all beacons" in
the ath(4) driver a few months ago. Trouble is, we now need the filtering
again.

Tested:

• ath(4), IBSS, on a very busy IBSS channel with lots (> 15) IBSS networks.

PR: kern/208643
Sponsored by: Eva Automation, Inc.