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: 550268ca

 ("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>