Add support in the driver ethtool ops to modify the NFP FEC modes.

The FEC modes can be set for vNIC associated with physical ports or
for MAC representor netdevs.
Signed-off-by: NDirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: NSimon Horman <simon.horman@netronome.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>