bridge: multicast: notify on group delete

Group notifications were not sent when a group expired or was deleted
due to bridge/port device being deleted. So add br_mdb_notify() to
br_multicast_del_pg().

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>