bridge: Always flood broadcast packets

As is_multicast_ether_addr returns true on broadcast packets as
well, we need to explicitly exclude broadcast packets so that
they're always flooded.  This wasn't an issue before as broadcast
packets were considered to be an unregistered multicast group,
which were always flooded.  However, as we now only flood such
packets to router ports, this is no longer acceptable.

Reported-by: Michael Guntsche <mike@it-loops.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>