bridge: Forward reserved group addresses if !STP

Make all frames sent to reserved group MAC addresses (01:80:c2:00:00:00 to
01:80:c2:00:00:0f) be forwarded if STP is disabled. This enables
forwarding EAPOL frames, among other things.

Signed-off-by: Benjamin Poirier <benjamin.poirier@polymtl.ca>
Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>