iio: dummy: iio_simple_dummy_buffer: use 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/20240718-dev-iio-masklength-private2-v1-4-8e12cd042906@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/dummy/iio_simple_dummy_buffer.c

@@ -68,7 +68,7 @@ static irqreturn_t iio_simple_dummy_trigger_h(int irq, void *p)
 	 * Here let's pretend we have random access. And the values are in the
 	 * constant table fakedata.
 	 */
-	for_each_set_bit(j, indio_dev->active_scan_mask, indio_dev->masklength)
+	iio_for_each_active_channel(indio_dev, j)
 		data[i++] = fakedata[j];
 
 	iio_push_to_buffers_with_timestamp(indio_dev, data,