mlxsw: spectrum_switchdev: Publish mlxsw_sp_switchdev_notifier

The notifier block will need to be passed to vxlan_fdb_replay() in a
follow-up patch.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>