iio: adc: stm32-dfsdm-adc: 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>
Link: https://patch.msgid.link/20240726-dev-iio-masklength-private3-v1-9-82913fc0fb87@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/stm32-dfsdm-adc.c

@@ -987,7 +987,7 @@ static int stm32_dfsdm_update_scan_mode(struct iio_dev *indio_dev,
 {
 	struct stm32_dfsdm_adc *adc = iio_priv(indio_dev);
 
-	adc->nconv = bitmap_weight(scan_mask, indio_dev->masklength);
+	adc->nconv = bitmap_weight(scan_mask, iio_get_masklength(indio_dev));
 	adc->smask = *scan_mask;
 
 	dev_dbg(&indio_dev->dev, "nconv=%d mask=%lx\n", adc->nconv, *scan_mask);