[S390] dcss: Fix Unlikely(x) != y

Fix Unlikely(x) != y

Cc: Gerald Schaefer <geraldsc@de.ibm.com>
Cc: Stefan Weinhuber <wein@de.ibm.com>
Cc: Carsten Otte <cotte@de.ibm.com>
Signed-off-by: Roel Kluin <12o3l@tiscali.nl>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>