rtc: ds1307: handle oscillator failure flags for ds1388 variant

The FLAG register is at a different location to the other supported RTCs
so this requires an extra case in the existing switch statement.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Link: https://lore.kernel.org/r/20200207031812.14424-2-chris.packham@alliedtelesis.co.nz


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>