sfxge(4): allow to use PHY stats on Huntington/Medford

EFSYS_OPT_PHY_STATS can be used with Huntington or Medford, not just Siena.

Submitted by: Mark Spender <mspender at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18116