net/mlx4: Verify port number in __mlx4_unregister_mac

Verify port number to avoid crashes if port number is outside the range.

Signed-off-by: Eli Cohen <eli@mellanox.com>
Signed-off-by: Eugenia Emantayev <eugenia@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>