net/mlx5e: Add support for FEC modes based on 50G per lane links

Introduce new FEC modes:
- RS-FEC-(544,514)
- LL_RS-FEC-(272,257+1)
Add support in ethtool for set and get callbacks for the new modes
above. While RS-FEC-(544,514) is mapped to exsiting RS FEC mode,
LL_RS-FEC-(272,257+1) is mapped to a new ethtool link mode: LL-RS.

Add support for FEC on 50G per lane link modes up to 400G. The new link
modes uses a u16 fields instead of u8 fields for the legacy link modes.

Signed-off-by: Aya Levin <ayal@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>