iio: accel: bmc150-accel-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-v1-7-98193bf536a6@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/accel/bmc150-accel-core.c

@@ -1007,8 +1007,7 @@ static int __bmc150_accel_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]));