rtc: mc146818-lib: fix signedness bug in mc146818_get_time()

The mc146818_get_time() function returns zero on success or negative
a error code on failure.  It needs to be type int.

Fixes: d35786b3 ("rtc: mc146818-lib: change return values of mc146818_get_time()")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Mateusz Jończyk <mat.jonczyk@o2.pl>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220111071922.GE11243@kili