rtc: pcf8563: Fix interrupt trigger method

The PCF8563 datasheet says the interrupt line is active low and stays
active until the events are cleared, i.e. a level trigger interrupt.

Fix the flags used to request the interrupt.

Fixes: ede3e9d4 ("drivers/rtc/rtc-pcf8563.c: add alarm support")
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>