sfxge(4): clarify port mode names and masks

New port mode names are defined for Medford2 and later, and
the existing names are aliased to them. Add comments with the
numeric port mode to clarify the external port modes table.

Submitted by: Andy Moreton <amoreton at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18199