IRQ type in ad_sigma_delta_info struct was missing.

In Sigma-Delta devices the SDO line is also used as an interrupt.
Leaving IRQ on level instead of falling might trigger a sample read
when the IRQ is enabled, as the SDO line is already low. Not sure
if SDO line will always immediately go high in ad_sd_buffer_postenable
before the IRQ is enabled.

Also the datasheet seem to explicitly say the falling edge of the SDO
should be used as an interrupt:
>From the AD7192 datasheet: "The DOUT/RDY falling edge can be used
as an interrupt to a processor,"

Fixes: da4d3d6b ("iio: adc: ad-sigma-delta: Allow custom IRQ flags")
Signed-off-by: Alexandru Tachici <alexandru.tachici@analog.com>
Cc: <Stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20210906065630.16325-2-alexandru.tachici@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>