rtc: ds1685: fix crash caused by referencing wrong device struct

sysfs entries added by rtc_add_group are called with the rtc device
as argument and not the underlying device. Fixed by using the dev->parent

Fixes: cfb74916 ("rtc: ds1685: use rtc_add_group")
Signed-off-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>