iio: accel: msa311: 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-v1-10-98193bf536a6@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/accel/msa311.c

@@ -900,8 +900,7 @@ static irqreturn_t msa311_buffer_thread(int irq, void *p)
 
 	mutex_lock(&msa311->lock);
 
-	for_each_set_bit(bit, indio_dev->active_scan_mask,
-			 indio_dev->masklength) {
+	iio_for_each_active_channel(indio_dev, bit) {
 		chan = &msa311_channels[bit];
 
 		err = msa311_get_axis(msa311, chan, &axis);