iio: imu: kmx61: 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>
Link: https://patch.msgid.link/20240718-dev-iio-masklength-private2-v1-11-8e12cd042906@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/imu/kmx61.c

@@ -1200,8 +1200,7 @@ static irqreturn_t kmx61_trigger_handler(int irq, void *p)
 		base = KMX61_MAG_XOUT_L;
 
 	mutex_lock(&data->lock);
-	for_each_set_bit(bit, indio_dev->active_scan_mask,
-			 indio_dev->masklength) {
+	iio_for_each_active_channel(indio_dev, bit) {
 		ret = kmx61_read_measurement(data, base, bit);
 		if (ret < 0) {
 			mutex_unlock(&data->lock);