iio: adc: mcp3911: 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-v1-4-82913fc0fb87@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/mcp3911.c

@@ -510,7 +510,7 @@ static irqreturn_t mcp3911_trigger_handler(int irq, void *p)
 		goto out;
 	}
 
-	for_each_set_bit(scan_index, indio_dev->active_scan_mask, indio_dev->masklength) {
+	iio_for_each_active_channel(indio_dev, scan_index) {
 		const struct iio_chan_spec *scan_chan = &indio_dev->channels[scan_index];
 
 		adc->scan.channels[i] = get_unaligned_be24(&adc->rx_buf[scan_chan->channel * 3]);