net/mlx5e: Add support for loopback selftest

Extend the self diagnostic tests to support loopback test.

The loopback test doesn't require the offline flag, it will use the
generic dev_queue_xmit and a dedicated packet_type to capture and verify
mlx5e selftest loopback packets.

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Kamal Heib <kamalh@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>