iio: adc: xilinx: Fix VREFN scale

commit a57f8dac upstream.

The scaling factor for VREFN is 3.0/4096 (not 1.0/4096), just as for
VREFP. This is not immediately obvious from the specification (Xilinx
UG480), but has been confirmed by Xilinx support.
Suggested-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Thomas Betker <thomas.betker@rohde-schwarz.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

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

@@ -857,6 +857,7 @@ static int xadc_read_raw(struct iio_dev *indio_dev,
 			case XADC_REG_VCCINT:
 			case XADC_REG_VCCAUX:
 			case XADC_REG_VREFP:
+			case XADC_REG_VREFN:
 			case XADC_REG_VCCBRAM:
 			case XADC_REG_VCCPINT:
 			case XADC_REG_VCCPAUX: