iio: adc: rtq6056: 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-v1-7-82913fc0fb87@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/rtq6056.c

@@ -643,7 +643,7 @@ static irqreturn_t rtq6056_buffer_trigger_handler(int irq, void *p)
 
 	pm_runtime_get_sync(dev);
 
-	for_each_set_bit(bit, indio_dev->active_scan_mask, indio_dev->masklength) {
+	iio_for_each_active_channel(indio_dev, bit) {
 		unsigned int addr = rtq6056_channels[bit].address;
 
 		ret = regmap_read(priv->regmap, addr, &raw);