fix KV initialization bug

Author: Richard Unger

src/BLDCMotor.cpp

@@ -44,7 +44,9 @@ BLDCMotor::BLDCMotor(int pp, float _R, float _KV, float _inductance)
   phase_resistance = _R;
   // save back emf constant KV = 1/KV
   // 1/sqrt(2) - rms value
-  KV_rating = _KV*_SQRT2;
+  KV_rating = NOT_SET;
+  if (_isset(_KV))
+    KV_rating = _KV*_SQRT2;
   // save phase inductance
   phase_inductance = _inductance;