staging:iio:ade7758: Fix check if channels are enabled in prenable

We should check if a channel is enabled, not if no channels are enabled.

Fixes: 550268ca1111 ("staging:iio: scrap scan_count and ensure all drivers use active_scan_mask")
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/staging/iio/meter/ade7758_ring.c

@@ -87,7 +87,7 @@ static int ade7758_ring_preenable(struct iio_dev *indio_dev)
 {
 	unsigned channel;
 
-	if (!bitmap_empty(indio_dev->active_scan_mask, indio_dev->masklength))
+	if (bitmap_empty(indio_dev->active_scan_mask, indio_dev->masklength))
 		return -EINVAL;
 
 	channel = find_first_bit(indio_dev->active_scan_mask,