trivial: rtc-stk17ta8: fix sparse warning

Fix this sparse warning:

  drivers/rtc/rtc-stk17ta8.c:403:2: warning: returning void-valued expression
Acked-by: NAlessandro Zummo <a.zummo@towertech.it>
Signed-off-by: NHannes Eder <hannes@hanneseder.net>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>

drivers/rtc/rtc-stk17ta8.c

@@ -400,7 +400,7 @@ static __init int stk17ta8_init(void)
 
 static __exit void stk17ta8_exit(void)
 {
-	return platform_driver_unregister(&stk17ta8_rtc_driver);
+	platform_driver_unregister(&stk17ta8_rtc_driver);
 }
 
 module_init(stk17ta8_init);