bridge: mdb: Passing the port-group pointer to br_mdb module

Passing the port-group to br_mdb in order to allow direct access to the
structure. br_mdb will later use the structure to reflect HW reflection
status via "state" variable.

Signed-off-by: Elad Raz <eladr@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>