sfxge(4): support improvements to bandwidth calculations

Change the interface to ef10_nic_get_port_mode_bandwidth()
so more NIC information can be used to infer bandwidth
requirements. Huntington calculations separated out
completely.

Submitted by: Richard Houldsworth <rhouldsworth at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18278