iio: adc: ti-ads8688: 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-v1-18-82913fc0fb87@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/ti-ads8688.c

@@ -384,9 +384,7 @@ static irqreturn_t ads8688_trigger_handler(int irq, void *p)
 	u16 buffer[ADS8688_MAX_CHANNELS + sizeof(s64)/sizeof(u16)] __aligned(8);
 	int i, j = 0;
 
-	for (i = 0; i < indio_dev->masklength; i++) {
-		if (!test_bit(i, indio_dev->active_scan_mask))
-			continue;
+	iio_for_each_active_channel(indio_dev, i) {
 		buffer[j] = ads8688_read(indio_dev, i);
 		j++;
 	}