rtc: wm831x: Add IRQF_ONESHOT flag

fix below issue reported by coccicheck
drivers/rtc/rtc-wm831x.c:436:7-32: ERROR: Threaded IRQ with no primary
handler requested without IRQF_ONESHOT

Signed-off-by: Hariprasad Kelam <hariprasad.kelam@gmail.com>
Acked-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20190711175615.GA13651@hari-Inspiron-1545
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>