rtc: abx80x: fix the RTC_VL_CLR clearing all status flags

rtc: abx80x: fix the RTC_VL_CLR clearing all status flags

The RTC_VL_CLR ioctl intends to clear only the battery low flag (BLF),
however the current implementation writes 0 to the status register,
clearing all status bits.

Fix this by writing back the masked status value so that only BLF is
cleared, preserving other status flags.

Fixes: ffe1c5a2d427 ("rtc: abx80x: Implement RTC_VL_READ,CLR ioctls")
Signed-off-by: Antoni Pokusinski <apokusinski01@gmail.com>
Link: https://patch.msgid.link/20260415160610.127155-2-apokusinski01@gmail.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>