net: bridge: split out the switchdev portion of br_mdb_notify

Similar to fdb_notify() and br_switchdev_fdb_notify(), split the
switchdev specific logic from br_mdb_notify() into a different function.
This will be moved later in br_switchdev.c.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Acked-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>