rtc: mc146818-lib: change return values of mc146818_get_time()

No function is checking mc146818_get_time() return values yet, so
correct them to make them more customary.

Signed-off-by: Mateusz Jończyk <mat.jonczyk@o2.pl>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20211210200131.153887-3-mat.jonczyk@o2.pl