Fixed DMA time comparison.

Change-Id: I9aa454f3a3c75bd2ad65c0bcb62591e1acbc17f8
diff --git a/aos/externals/forwpilib/dma.cc b/aos/externals/forwpilib/dma.cc
index 3d41c43..ce8bbd3 100644
--- a/aos/externals/forwpilib/dma.cc
+++ b/aos/externals/forwpilib/dma.cc
@@ -350,8 +350,12 @@
 
 ssize_t DMASample::offset(int index) const { return dma_->channel_offsets_[index]; }
 
+uint32_t DMASample::GetTime() const {
+  return read_buffer_[dma_->capture_size_ - 1];
+}
+
 double DMASample::GetTimestamp() const {
-  return static_cast<double>(read_buffer_[dma_->capture_size_ - 1]) * 0.000001;
+  return static_cast<double>(GetTime()) * 0.000001;
 }
 
 bool DMASample::Get(DigitalSource *input) const {