Add FEC API to netlink.

This is not a 1-to-1 conversion.

FEC settings already depend on link modes to tell user which
modes are supported. Take this further an use link modes for
manual configuration. Old struct ethtool_fecparam is still
used to talk to the drivers, so we need to translate back
and forth. We can revisit the internal API if number of FEC
encodings starts to grow.

Enforce only one active FEC bit (by using a bit position
rather than another mask).
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>