iio: adc: ad_sigma_delta: use new '.masklength' accessors

iio: adc: ad_sigma_delta: use new '.masklength' accessors

Make use of iio_get_masklength) and iio_for_each_active_channel() to
access '.masklength' so it can be annotated as __private when there
are no more direct users of it.

Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Reviewed-by: Alexandru Ardelean <aardelean@baylibre.com>
Link: https://patch.msgid.link/20240702-dev-iio-masklength-private-v1-17-98193bf536a6@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>