rtc: ds1307: Clear OSF flag on DS1388 when setting time

Ensure the OSF flag is cleared on the DS1388 when the clock is set.

Fixes: df11b323 ("rtc: ds1307: handle oscillator failure flags for ds1388 variant")
Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20200818013543.4283-1-chris.packham@alliedtelesis.co.nz