[S390] css: fix sparse warning

fix this sparse warning:

drivers/s390/cio/css.c:580:6: warning: symbol 'css_schedule_eval_all_unreg'
was not declared. Should it be static?
Signed-off-by: NSebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/cio/css.c

@@ -577,7 +577,7 @@ static int __unset_registered(struct device *dev, void *data)
 	return 0;
 }
 
-void css_schedule_eval_all_unreg(void)
+static void css_schedule_eval_all_unreg(void)
 {
 	unsigned long flags;
 	struct idset *unreg_set;