drivers/rtc/rtc-ab8500.c: use IRQF_ONESHOT when requesting a threaded IRQ

This driver's IRQ registration is failing because the kernel now forces
IRQs to be ONESHOT if no IRQ handler is passed.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>