iio: light: isl29125: 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-v1-14-8e12cd042906@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/light/isl29125.c

@@ -181,8 +181,7 @@ static irqreturn_t isl29125_trigger_handler(int irq, void *p)
 	struct isl29125_data *data = iio_priv(indio_dev);
 	int i, j = 0;
 
-	for_each_set_bit(i, indio_dev->active_scan_mask,
-		indio_dev->masklength) {
+	iio_for_each_active_channel(indio_dev, i) {
 		int ret = i2c_smbus_read_word_data(data->client,
 			isl29125_regs[i].data);
 		if (ret < 0)