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>