iio: adc: ad7266: 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-v1-14-98193bf536a6@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/ad7266.c

@@ -123,7 +123,8 @@ static int ad7266_update_scan_mode(struct iio_dev *indio_dev,
 	const unsigned long *scan_mask)
 {
 	struct ad7266_state *st = iio_priv(indio_dev);
-	unsigned int nr = find_first_bit(scan_mask, indio_dev->masklength);
+	unsigned int nr = find_first_bit(scan_mask,
+					 iio_get_masklength(indio_dev));
 
 	ad7266_select_input(st, nr);