iio: adc: xilinx: Fix VREFP scale

The scaling factor for VREFP is 3.0/4096, not 1.0/4096; fix this to get
correct readings.

Signed-off-by: Thomas Betker <thomas.betker@rohde-schwarz.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
diff --git a/drivers/iio/adc/xilinx-xadc-core.c b/drivers/iio/adc/xilinx-xadc-core.c
index 0ad7b50..6fa629b 100644
--- a/drivers/iio/adc/xilinx-xadc-core.c
+++ b/drivers/iio/adc/xilinx-xadc-core.c
@@ -856,6 +856,7 @@
 			switch (chan->address) {
 			case XADC_REG_VCCINT:
 			case XADC_REG_VCCAUX:
+			case XADC_REG_VREFP:
 			case XADC_REG_VCCBRAM:
 			case XADC_REG_VCCPINT:
 			case XADC_REG_VCCPAUX: