iio: adc: ad7124: Set IRQ type to falling

Ad7124 data-sheet specifies that the falling edge
of the DOUT line should be used for an interrupt.
The current irq flag (IRQF_TRIGGER_LOW) used will
cause unwanted behaviour. When enabling the interrupt
it will fire once because the DOUT line is already low.
This will make the driver to read an unfinished conversion
from the chip.

This patch sets the irq type to the one specified in
the data-sheet.

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