bridge: mcast: Simplify MDB entry creation

Before creating a new MDB entry, br_multicast_new_group() will call
br_mdb_ip_get() to see if one exists and return it if so.

Therefore, simply call br_multicast_new_group() and omit the call to
br_mdb_ip_get().

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>