rtc: pcf8523: don't return invalid date when battery is low

Hi stable kernel team,

Please consider upstream commit ecb4a353d3afd45 ("rtc: pcf8523: don't return invalid date when battery is low") for stable backport. As the commit log explains, this fixes bogus system date when RTC backup battery is low. With this fix date is set to the well known 01-01-1970 value instead.

This commit should be applicable to v4.9+.

Thanks, baruch

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