Correctly wrap the producer queue index when dequeuing commands. This wasn't

a problem for command responses since we rarely ever filled the queue.
However, adapter-initiated commands have a much smaller queue and could
tickle this bug.  It's possible that this might fix the recently reported
problems with the aac-2120s, though I haven't been able to reproduce the
problem locally.

MFC-After: 1 day

sys/dev/aac/aac.c

@@ -1877,7 +1877,11 @@ aac_dequeue_fib(struct aac_softc *sc, int queue, u_int32_t *fib_size,
 		error = ENOENT;
 		goto out;
 	}
-	
+
+	/* wrap the pi so the following test works */
+	if (pi >= aac_qinfo[queue].size)
+		pi = 0;
+
 	notify = 0;
 	if (ci == pi + 1)
 		notify++;