iio: adc: ti-ads131e08: make use of iio_for_each_active_channel()

Use iio_for_each_active_channel() to iterate over active channels
accessing '.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-v1-17-82913fc0fb87@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/ti-ads131e08.c

@@ -637,7 +637,7 @@ static irqreturn_t ads131e08_trigger_handler(int irq, void *private)
 	if (ret)
 		goto out;
 
-	for_each_set_bit(chn, indio_dev->active_scan_mask, indio_dev->masklength) {
+	iio_for_each_active_channel(indio_dev, chn) {
 		src = st->rx_buf + ADS131E08_NUM_STATUS_BYTES + chn * num_bytes;
 		dest = st->tmp_buf.data + i * ADS131E08_NUM_STORAGE_BYTES;