s390/qdio: remove unneeded siga-sync for Output Queue

get_outbound_buffer_frontier() is only reached via qdio_inspect_queue(),
and there we already call qdio_siga_sync_q() unconditionally.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Reviewed-by: Benjamin Block <bblock@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>