rtc: sysfs: use kobj_to_dev

use kobj_to_dev instead of open-conding it

Signed-off-by: suguosong <suguosong@xiaomi.com>
Link: https://lore.kernel.org/r/20200225021923.8570-1-guosongsu@gmail.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>