iio: imu: adis: Add irq flag variable

There are some ADIS devices that can configure the data ready pin
polarity. Hence, we cannot hardcode our IRQ mask as IRQF_TRIGGER_RISING
since we might want to have it as IRQF_TRIGGER_FALLING.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>