iio: imu: adis16480: 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-v1-20-98193bf536a6@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/imu/adis16480.c

@@ -1395,7 +1395,7 @@ static irqreturn_t adis16480_trigger_handler(int irq, void *p)
 		goto irq_done;
 	}
 
-	for_each_set_bit(bit, indio_dev->active_scan_mask, indio_dev->masklength) {
+	iio_for_each_active_channel(indio_dev, bit) {
 		/*
 		 * When burst mode is used, temperature is the first data
 		 * channel in the sequence, but the temperature scan index