Fixed DMA time comparison.

Change-Id: I9aa454f3a3c75bd2ad65c0bcb62591e1acbc17f8
diff --git a/aos/externals/forwpilib/dma.h b/aos/externals/forwpilib/dma.h
index 754b0a5..2f5b331 100644
--- a/aos/externals/forwpilib/dma.h
+++ b/aos/externals/forwpilib/dma.h
@@ -22,8 +22,10 @@
  public:
   DMASample() = default;
 
-  // Returns the FPGA timestamp of the sample.
+  // Returns the FPGA timestamp of the sample in seconds.
   double GetTimestamp() const;
+  // Returns the FPGA timestamp of the sample in microseconds.
+  uint32_t GetTime() const;
 
   // All Get methods either return the requested value, or set the Error.