i40e: Add support FEC configuration for Fortville 25G

This patch adds support for setting/getting FEC configuration
using ethtool options:
       set/show-priv-flags rs-fec/base-r-fec
       set/show-fec off/rs/baser/auto for kernels version >= 4.14

Signed-off-by: Damian Dybek <damian.dybek@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>