mac80211: as a 4-addr station, do not receive packets for other stations

Since 4-addr frames completely override the source address which will
make it into the converted 802.3 frames, receiving frames for other
4-addr stations will confuse the bridging code.

To be able to handle traffic for all connected devices, the bridge
code will automatically turn on promiscuous mode, which triggers
this problem.

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