iio: core: make use of iio_get_masklength()

Use iio_get_masklength() 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-v1-2-98193bf536a6@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/industrialio-core.c

@@ -1912,7 +1912,7 @@ static void iio_sanity_check_avail_scan_masks(struct iio_dev *indio_dev)
 	int i;
 
 	av_masks = indio_dev->available_scan_masks;
-	masklength = indio_dev->masklength;
+	masklength = iio_get_masklength(indio_dev);
 	longs_per_mask = BITS_TO_LONGS(masklength);
 
 	/*