sfxge(4): support Medford2 event timer semantics

The event timer interface has changed for Medford2 - for
details see bug66418 comment 9. Update the common code to
use the new timer semantics for Medford2.

Submitted by: Andy Moreton <amoreton at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18198