iio: health: afe4403: 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/20240718-dev-iio-masklength-private2-v1-5-8e12cd042906@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/health/afe4403.c

@@ -321,8 +321,7 @@ static irqreturn_t afe4403_trigger_handler(int irq, void *private)
 	if (ret)
 		goto err;
 
-	for_each_set_bit(bit, indio_dev->active_scan_mask,
-			 indio_dev->masklength) {
+	iio_for_each_active_channel(indio_dev, bit) {
 		ret = spi_write_then_read(afe->spi,
 					  &afe4403_channel_values[bit], 1,
 					  rx, sizeof(rx));