ethtool: Add support for low latency RS FEC

Add support for low latency Reed Solomon FEC as LLRS.

The LL-FEC is defined by the 25G/50G ethernet consortium,
in the document titled "Low Latency Reed Solomon Forward Error Correction"

Signed-off-by: Aya Levin <ayal@mellanox.com>
Reviewed-by: Eran Ben Elisha <eranbe@mellanox.com>
CC: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>