net/mlx5e: Enable local loopback in loopback selftest

Before running the ethtool's loopback selftest, we need
to make sure that the local loopback is enabled.

Signed-off-by: Huy Nguyen <huyn@mellanox.com>
Reviewed-by: Daniel Jurgens <danielj@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>