sfxge(4): add MCDI agnostic wrapper for MAC stats clear

If a libefx-based driver needs some way to clear port statistics,
then an MCDI agnostic method is required.

Submitted by: Ivan Malov <ivan.malov at oktetlabs.ru>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18064