iio: adc: ad_sigma_delta: allow overwriting the IRQ flags

Make sure we can specify the IRQ trigger type from firmware and drivers
won't ignore it. In fact, this how it should be done but since someone
might be already depending on the driver to hardcode the trigger type
(and not specifying it in firmware), let's do it like this so there's
no possible breakage.

Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20240117-dev_sigma_delta_no_irq_flags-v1-2-db39261592cf@analog.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>