iio: frequency: cf_axi_dds: Use iio_get_masklength()

Upstream has removed direct access to indio_dev->masklength.
Replace direct access with the iio_get_masklength() helper function
to align with this change and to avoid warnings from the sparse
checker.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>

Author: nunojsa

drivers/iio/frequency/cf_axi_dds.c

@@ -1041,7 +1041,7 @@ static int cf_axi_dds_update_scan_mode(struct iio_dev *indio_dev,
 	struct cf_axi_dds_state *st = iio_priv(indio_dev);
 	unsigned int i, sel;
 
-	for (i = 0; i < indio_dev->masklength; i++) {
+	for (i = 0; i < iio_get_masklength(indio_dev); i++) {
 
 		if (test_bit(i, scan_mask))
 			sel = DATA_SEL_DMA;