mac80211: fix broadcast frame handling for 4-addr AP VLANs

Without this patch, broadcast frames from the station behind a
4-addr AP VLAN would be reflected back to the source.
Fix this by checking the 4-addr flag before bridging multicast
frames in the cell.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>