bridge: add missing vid to br_mdb_get()

Obviously, vid should be considered when searching for multicast
group.

Cc: Vlad Yasevich <vyasevic@redhat.com>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
Acked-by: Vlad Yasevich <vyasevich@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>