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

While at it, remove some unneeded line breaks.
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-v1-3-98193bf536a6@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>