iio: adc: ad7780: Fix IRQ flag

Correct IRQ flag here is falling.

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 AD7780 datasheet: " The DOUT/Figure 22 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/20210906065630.16325-3-alexandru.tachici@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>