rtc: mc146818: Dont test for bit 0-5 in Register D

The recent change to validate the RTC turned out to be overly tight.

While it cures the problem on the reporters machine it breaks machines
with Intel chipsets which use bit 0-5 of the D register. So check only
for bit 6 being 0 which is the case on these Intel machines as well.

Fixes: 211e5db1 ("rtc: mc146818: Detect and handle broken RTCs")
Reported-by: Serge Belyshev <belyshev@depni.sinp.msu.ru>
Reported-by: Dirk Gouders <dirk@gouders.net>
Reported-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Dirk Gouders <dirk@gouders.net>
Tested-by: Len Brown <len.brown@intel.com>
Tested-by: Borislav Petkov <bp@suse.de>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/87zh0nbnha.fsf@nanos.tec.linutronix.de