net/mlx5e: Ethtool driver callback for query/set FEC policy

Driver callback function for 'ethtool --show-fec',
'ethtool --set-fec' commands.

The query function returns active and configured FEC policy
for current link speed.

The set function sets FEC policy for all supported link
speeds.
1) If current link speed doesn't support requested FEC policy,
   the function fails.
2) If a different link speed doesn't support requested FEC
   policy, FEC capbilities for this speed are turned off.

Signed-off-by: Shay Agroskin <shayag@mellanox.com>
Reviewed-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>