iio: adc: mt6360-adc: 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>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://patch.msgid.link/20240726-dev-iio-masklength-private3-v1-5-82913fc0fb87@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/mt6360-adc.c

@@ -268,7 +268,7 @@ static irqreturn_t mt6360_adc_trigger_handler(int irq, void *p)
 	int i = 0, bit, val, ret;
 
 	memset(&data, 0, sizeof(data));
-	for_each_set_bit(bit, indio_dev->active_scan_mask, indio_dev->masklength) {
+	iio_for_each_active_channel(indio_dev, bit) {
 		ret = mt6360_adc_read_channel(mad, bit, &val);
 		if (ret < 0) {
 			dev_warn(&indio_dev->dev, "Failed to get channel %d conversion val\n", bit);