net/mlx5e: Add support for ethtool self diagnostics test

The self diagnostics test implementaion include the following features:
1. Link Test: Check that link is in up state.
2. Speed Test: Check that link was negotiated correctly.
3. Health Test: Check the device health.

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