iio: adc: max1118: 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-v1-3-82913fc0fb87@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/max1118.c

@@ -174,8 +174,7 @@ static irqreturn_t max1118_trigger_handler(int irq, void *p)
 
 	mutex_lock(&adc->lock);
 
-	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];
 		int ret = max1118_read(indio_dev, scan_chan->channel);