nfp: implement ethtool FEC mode settings

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: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>