rtc: rv3028: fix PORF handling

The PORF bit is cleared on interrupts which prevents the driver to know
when the time and date are invalid. Stop clearing PORF in the interrupt
handler.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210126221435.2152339-1-alexandre.belloni@bootlin.com