rtc: gamecube: Report low battery as invalid data

I haven’t been able to test this patch as all of my consoles have a
working RTC battery, but according to the documentation it should work
like that.

Signed-off-by: Emmanuel Gil Peyrot <linkmauve@linkmauve.fr>
Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20211215175501.6761-3-linkmauve@linkmauve.fr