iio: adc: ad799x: 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-v1-16-98193bf536a6@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/ad799x.c

@@ -237,7 +237,8 @@ static int ad799x_update_scan_mode(struct iio_dev *indio_dev,
 	if (!st->rx_buf)
 		return -ENOMEM;
 
-	st->transfer_size = bitmap_weight(scan_mask, indio_dev->masklength) * 2;
+	st->transfer_size = bitmap_weight(scan_mask,
+					  iio_get_masklength(indio_dev)) * 2;
 
 	switch (st->id) {
 	case ad7992: