nfp: add support for reporting active FEC mode

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>