sfxge: support MAC spoofing for 4.2.x firmare

Common code should infer other privileges from Admin privilege to
support firmware that pre-dates introduction of specific privilege
flags.

Submitted by: Richard Houldsworth <rhouldsworth at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D4327