iio: adc: rockchip_saradc: 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>
Acked-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patch.msgid.link/20240726-dev-iio-masklength-private3-v1-6-82913fc0fb87@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/rockchip_saradc.c

@@ -370,7 +370,7 @@ static irqreturn_t rockchip_saradc_trigger_handler(int irq, void *p)
 
 	mutex_lock(&info->lock);
 
-	for_each_set_bit(i, i_dev->active_scan_mask, i_dev->masklength) {
+	iio_for_each_active_channel(i_dev, i) {
 		const struct iio_chan_spec *chan = &i_dev->channels[i];
 
 		ret = rockchip_saradc_conversion(info, chan);