iio: accel: fxls8962af-core: 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-v1-9-98193bf536a6@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/accel/fxls8962af-core.c

@@ -966,8 +966,7 @@ static int fxls8962af_fifo_flush(struct iio_dev *indio_dev)
 		int j, bit;
 
 		j = 0;
-		for_each_set_bit(bit, indio_dev->active_scan_mask,
-				 indio_dev->masklength) {
+		iio_for_each_active_channel(indio_dev, bit) {
 			memcpy(&data->scan.channels[j++], &buffer[i * 3 + bit],
 			       sizeof(data->scan.channels[0]));
 		}