s390/ccwgroup: increase string buffer size

Avoid false positive warnings like this with gcc 7.1:

drivers/s390/cio/ccwgroup.c:41:21:
 warning: '%d' directive writing between 1 and 10 bytes into a region of size 4
   sprintf(str, "cdev%d", i);

and simply increase the size of the string buffer.
Reviewed-by: NSebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: NHeiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/cio/ccwgroup.c

@@ -35,7 +35,7 @@ static struct bus_type ccwgroup_bus_type;
 static void __ccwgroup_remove_symlinks(struct ccwgroup_device *gdev)
 {
 	int i;
-	char str[8];
+	char str[16];
 
 	for (i = 0; i < gdev->count; i++) {
 		sprintf(str, "cdev%d", i);
@@ -238,7 +238,7 @@ static void ccwgroup_release(struct device *dev)
 
 static int __ccwgroup_create_symlinks(struct ccwgroup_device *gdev)
 {
-	char str[8];
+	char str[16];
 	int i, rc;
 
 	for (i = 0; i < gdev->count; i++) {