sfxge(4): set moderation in efx_ev_qcreate

This simplifies setting an initial interrupt moderation value, and
avoids most calls to evx_ev_qmoderate from contexts where MCDI is
not allowed (MCDI is need for an EVQ timer workaround in a later patch).

Submitted by: Andy Moreton <amoreton at solarflare.com>
Reviewed by: gnn
Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D6673