mlxsw: spectrum_switchdev: Consider mrouter status for mdb changes

When a mrouter is registered or leaves a mid, don't update the HW.

Signed-off-by: Nogah Frankel <nogahf@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>