The latest management firmware can now report the active FEC
mode. Adapt driver accordingly so that user can get the active
FEC mode by running command:

  # ethtool --show-fec <intf>

Also correct use of `fec` field.
Signed-off-by: Yinjun Zhang <yinjun.zhang@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>