net80211: do not enforce promiscuous mode for AP scan

Drivers should set their own filters via ic_scan_start()/ic_scan_end()
callbacks; and we don't need frames other than beacons or probe responses.

(Note: this was a noop since r287197 due to promiscuous mode with bridge
workaround)

Tested with Intel 3945BG, RTL8188EU and WUSB54GC in HOSTAP mode.

Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D5474