sfxge(4): let caller know that queue is already flushed

Tx/Rx queue may be already flushed due to Tx/Rx error on the queue or
MC reboot. Caller needs to know that the queue is already flushed to
avoid waiting for flush done event.

Submitted by: Andy Moreton <amoreton at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D18070