sfxge(4): update external port mapping for Medford

Extend the mapping table for external port numbering to support port modes
which output to the second external port only. Where supported, map from
the current port mode rather than inferring from all the available modes.
Updated comments for clarity.

Submitted by: Richard Houldsworth <rhouldsworth at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D8210