trivial: rtc-ds1511: fix sparse warning

Fix this sparse warning:

  drivers/rtc/rtc-ds1511.c:634:2: warning: returning void-valued expression

Acked-by: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/rtc/rtc-ds1511.c

@@ -631,7 +631,7 @@ ds1511_rtc_init(void)
  static void __exit
 ds1511_rtc_exit(void)
 {
-	return platform_driver_unregister(&ds1511_rtc_driver);
+	platform_driver_unregister(&ds1511_rtc_driver);
 }
 
 module_init(ds1511_rtc_init);