Wsign-compare: Fixed all sign compare warnings in src

This commit addresses all warnings emitted from -Wunused-function
in the C++ src.

Signed-off-by: Noel Eck <noel.eck@intel.com>

src/adxrs610/adxrs610.cxx

@@ -66,7 +66,7 @@ float ADXRS610::calibrateZeroPoint(unsigned int samples)
   // The gyro should be in a stable, non-moving state
 
   float sum = 0;
-  for (int i=0; i<samples; i++)
+  for (unsigned int i=0; i<samples; i++)
     sum += getDataVolts();
 
   return sum / samples;