Fixed DMA on the roboRIO.

Change-Id: Ib4f2e9dfb78640ea8d28e62528a9296962bd2cd9
diff --git a/aos/externals/forwpilib/dma.h b/aos/externals/forwpilib/dma.h
index 446ded4..754b0a5 100644
--- a/aos/externals/forwpilib/dma.h
+++ b/aos/externals/forwpilib/dma.h
@@ -34,7 +34,7 @@
   // Returns the {1, 2, or 4} X scaled value of the encoder in the sample.
   int32_t Get(Encoder *input) const;
   // Returns the raw 12-bit value from the ADC.
-  int16_t GetValue(AnalogInput *input) const;
+  uint16_t GetValue(AnalogInput *input) const;
   // Returns the scaled value of an analog input.
   float GetVoltage(AnalogInput *input) const;