sfxge(4): cleanup: remove last use of deprecated function flags with privilege check

The function flags were changed to mirror the privileges, but
the privileges are preferred.

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/D8936