iio: accel: stk8312: 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>
Reviewed-by: Alexandru Ardelean <aardelean@baylibre.com>
Link: https://patch.msgid.link/20240702-dev-iio-masklength-private-v1-12-98193bf536a6@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/accel/stk8312.c

@@ -448,8 +448,7 @@ static irqreturn_t stk8312_trigger_handler(int irq, void *p)
 			goto err;
 		}
 	} else {
-		for_each_set_bit(bit, indio_dev->active_scan_mask,
-				 indio_dev->masklength) {
+		iio_for_each_active_channel(indio_dev, bit) {
 			ret = stk8312_read_accel(data, bit);
 			if (ret < 0) {
 				mutex_unlock(&data->lock);