iio: adc: ad-sigma-delta: Allow custom IRQ flags

Before this patch the ad_sigma_delta implementation hardcoded
the irq trigger type to low, assuming that all Sigma-Delta ADCs
have the same interrupt-type.

This patch allows all drivers using the ad_sigma_delta layer to set the
irq trigger type to the one specified in the datasheet.

Signed-off-by: Alexandru Tachici <alexandru.tachici@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>