sfxge(4): support FEC mode settings

Medford2 controllers support control and reporting of
FEC modes for 25G and higher links. See SF-109306-TC
for suggested usage in client code.

Submitted by: Andy Moreton <amoreton at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18197