ethtool: Add support for 400Gbps (50Gbps per lane) link modes

Add support for 400Gbps speed, link modes of 50Gbps per lane

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>