sfxge(4): fix Medford timer quantum calculation in common code

The event/timer block used sysclk in Huntington, but has been
moved to the dpcpu clock domain for Medford. Fix the computed
timer quantum to use the right clock.

Submitted by: Andy Moreton <amoreton at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D6389