net: bridge: allow add/remove permanent mdb entries on disabled ports

Adding mdb entries on disabled ports allows you to do setup before
accepting any traffic, avoiding any time where the port is not in the
multicast group.

Signed-off-by: Casper Andersson <casper.casan@gmail.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>