sfxge: use NIC config in place of some Huntington specific PIO constants

This should allow these functions to work for Medford as well.

Submitted by: Mark Spender <mspender at solarflare.com>
Reviewed by: gnn
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D4866