s390/qdio: fix atomic_sub() misusage

get_inbound_buffer_frontier() makes use of the return value of atomic_sub()
which shouldn't work, since atomic_sub() is supposed to return void.
This only works on s390 because atomic_sub() gets mapped to atomic_sub_return()
with a define without changing it's return value to void.

So use atomic_sub_return() instead of atomic_sub() in qeth code before fixing
atomic ops.
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/cio/qdio_main.c

@@ -528,7 +528,7 @@ static int get_inbound_buffer_frontier(struct qdio_q *q)
 	case SLSB_P_INPUT_PRIMED:
 		inbound_primed(q, count);
 		q->first_to_check = add_buf(q->first_to_check, count);
-		if (atomic_sub(count, &q->nr_buf_used) == 0)
+		if (atomic_sub_return(count, &q->nr_buf_used) == 0)
 			qperf_inc(q, inbound_queue_full);
 		if (q->irq_ptr->perf_stat_enabled)
 			account_sbals(q, count);