iio: adc: ad7791: fix IRQ flags

The interrupt is triggered on the falling edge rather than being a level
low interrupt.

Fixes: da4d3d6b

 ("iio: adc: ad-sigma-delta: Allow custom IRQ flags")
Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20230120124645.819910-1-nuno.sa@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>