[S390] Fix sparse warnings.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
diff --git a/drivers/s390/cio/cio.c b/drivers/s390/cio/cio.c
index e36ca27..89320c1 100644
--- a/drivers/s390/cio/cio.c
+++ b/drivers/s390/cio/cio.c
@@ -798,7 +798,7 @@
 cio_get_console_subchannel(void)
 {
 	if (!console_subchannel_in_use)
-		return 0;
+		return NULL;
 	return &console_subchannel;
 }