mlxsw: spectrum: Implement loopback ethtool feature

Allow user to enable loopback feature for individual ports using ethtool.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>