trivial: rtc-stk17ta8: fix sparse warning

Fix this sparse warning:

  drivers/rtc/rtc-stk17ta8.c:403:2: 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>