Older versions of Libreboot and Coreboot had an invalid value
(`3' in my case) in the century byte affecting the GM45 in
the Thinkpad X200. Not everybody's updated their firmwares,
and Linux <= 4.2 was able to read the RTC without problems,
so workaround this by ignoring invalid values.
Fixes: 3c217e51d8a272b9 ("rtc: cmos: century support") Cc: Alexandre Belloni <alexandre.belloni@bootlin.com> Cc: Alessandro Zummo <a.zummo@towertech.it> Cc: Sylvain Chouleur <sylvain.chouleur@intel.com> Cc: Patrick McDermott <patrick.mcdermott@libiquity.com> Cc: linux-rtc@vger.kernel.org Signed-off-by: Eric Wong <e@80x24.org> Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>