iio: adc: xilinx: Fix VREFN sign

The VREFN channel is bipolar, not unipolar. Small negative values do
occur (e.g., -1mV), and unsigned conversion maps them incorrectly to
large positive values (about +1V), so fix this.
Signed-off-by: NThomas Betker <thomas.betker@rohde-schwarz.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>

drivers/iio/adc/xilinx-xadc-core.c

@@ -997,7 +997,7 @@ static const struct iio_event_spec xadc_voltage_events[] = {
 	.num_event_specs = (_alarm) ? ARRAY_SIZE(xadc_voltage_events) : 0, \
 	.scan_index = (_scan_index), \
 	.scan_type = { \
-		.sign = 'u', \
+		.sign = ((_addr) == XADC_REG_VREFN) ? 's' : 'u', \
 		.realbits = 12, \
 		.storagebits = 16, \
 		.shift = 4, \