rtc: rtc-mc146818-lib: Use is_leap_year instead of calculate leap years

The is_leap_year() for determining leap year is provided in rtc lib.
This uses is_leap_year() instead of its own leap year determination
routine.

Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Acked-by: Maciej W. Rozycki <macro@orcam.me.uk>
Link: https://lore.kernel.org/r/20241007233618.1442937-1-nobuhiro1.iwamatsu@toshiba.co.jp


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