iio: accel: fxls8962af-core: make use of iio_for_each_active_channel()

iio: accel: fxls8962af-core: 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: Alexandru Ardelean <aardelean@baylibre.com>
Link: https://patch.msgid.link/20240702-dev-iio-masklength-private-v1-9-98193bf536a6@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>