iio: adc: mt6360-adc: make use of iio_for_each_active_channel()

iio: adc: mt6360-adc: make use of iio_for_each_active_channel()

Use iio_for_each_active_channel() to iterate over active channels
accessing '.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: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://patch.msgid.link/20240726-dev-iio-masklength-private3-v1-5-82913fc0fb87@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>