sfc: Fix search for flush completion events

When flushing queues we disable normal interrupt and event handling and
poll event queue 0 looking for flush completions.  Unfortunately the
flush event polling loop fails to move past any other type of event.
This problem has not been observed in production hardware but appears to
be a possibility.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>