ethtool: Added support for 50Gbps per lane link modes

Added support for 50Gbps per lane link modes. Define various 50G, 100G
and 200G link modes using it.

Signed-off-by: Aya Levin <ayal@mellanox.com>
Reviewed-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>