net/mlx4_en: Deregister multicast vxlan steering rules when going down

When mlx4_en_stop_port() is called, we need to deregister also the
tunnel steering rules that relate to multicast.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>