sfxge(4): update external port number calculation

Revise the external port calculation to support all
X2 port modes. The previous algorithm could not
handle different port numbering schemes on each cage.

Submitted by: Richard Houldsworth <rhouldsworth at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18285