sfxge(4): infer port mode bandwidth from max link speed

Limit the port mode bandwidth calculations by the maximum
reported link speed. This system detects 25G vs 10G cards,
and 100G port modes vs 40G.

Submitted by: Richard Houldsworth <rhouldsworth at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18279