rtc: remove a warning during scripts/kernel-doc step

During compilation using W=1 one would get:

drivers/rtc/systohc.c:11: info: Scanning doc for rtc_set_ntp_time
drivers/rtc/systohc.c:23: warning: bad line:  (
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/systohc.c

@@ -20,7 +20,7 @@
  * cases.
  *
  * -EPROTO is returned if now.tv_nsec is not close enough to *target_nsec.
- (
+ *
  * If temporary failure is indicated the caller should try again 'soon'
  */
 int rtc_set_ntp_time(struct timespec64 now, unsigned long *target_nsec)