rtc: cmos: ignore bogus century byte

BugLink: https://bugs.launchpad.net/bugs/1864773

[ Upstream commit 2a4daadd ]

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: 3c217e51 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

include/asm-generic/rtc.h

@@ -106,7 +106,7 @@ static inline unsigned int __get_rtc_time(struct rtc_time *time)
 	time->tm_year += real_year - 72;
 #endif
 
-	if (century)
+	if (century > 20)
 		time->tm_year += (century - 19) * 100;
 
 	/*