s390/qdio: restrict buffer merging to eligible devices

Only attempt to merge PENDING into EMPTY buffers for devices where
the PENDING state is actually expected (ie. IQD with CQ).
This might speed up the hot path a little bit.
Signed-off-by: NJulian Wiedmann <jwi@linux.vnet.ibm.com>
Reviewed-by: NUrsula Braun <ubraun@linux.vnet.ibm.com>
Reviewed-by: NBenjamin Block <bblock@linux.vnet.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/cio/qdio_main.c

@@ -761,7 +761,8 @@ static int get_outbound_buffer_frontier(struct qdio_q *q)
 	if (!count)
 		goto out;
 
-	count = get_buf_states(q, q->first_to_check, &state, count, 0, 1);
+	count = get_buf_states(q, q->first_to_check, &state, count, 0,
+			       q->u.out.use_cq);
 	if (!count)
 		goto out;