net/mlx4_en: Run loopback test only when port is up

Loopback can't work when port is down.

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>