[S390] qdio: prevent handling of buffers if count is zero

Exit do_QDIO early if the buffer count is zero to prevent side effects
in the following functions.
Signed-off-by: NJan Glauber <jang@linux.vnet.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/cio/qdio_main.c

@@ -1508,7 +1508,8 @@ int do_QDIO(struct ccw_device *cdev, unsigned int callflags,
 
 	if (irq_ptr->state != QDIO_IRQ_STATE_ACTIVE)
 		return -EBUSY;
-
+	if (!count)
+		return 0;
 	if (callflags & QDIO_FLAG_SYNC_INPUT)
 		return handle_inbound(irq_ptr->input_qs[q_nr],
 				      callflags, bufnr, count);