mfd: Enable IRQF_ONESHOT when requesting a threaded IRQ for ab8500gpadc

The kernel now forces IRQs to be ONESHOT if no IRQ handler is passed.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>