rtc: cmos: Refactor code by using the new dmi_get_bios_year() helper

Refactor code by using the new dmi_get_bios_year() helper instead of
open coding its functionality. This also makes logic slightly clearer.

No changes intended.

Cc: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Guilherme G. Piccoli <gpiccoli@canonical.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20200123131437.28157-3-andriy.shevchenko@linux.intel.com


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