iio: proximity: sx9500: 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-v1-21-8e12cd042906@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/proximity/sx9500.c

@@ -654,8 +654,7 @@ static irqreturn_t sx9500_trigger_handler(int irq, void *private)
 
 	mutex_lock(&data->mutex);
 
-	for_each_set_bit(bit, indio_dev->active_scan_mask,
-			 indio_dev->masklength) {
+	iio_for_each_active_channel(indio_dev, bit) {
 		ret = sx9500_read_prox_data(data, &indio_dev->channels[bit],
 					    &val);
 		if (ret < 0)