[wpilib] Call set(0) rather than disable for stopMotor (#4763)

For PWM motor controllers, this is going to be the fastest way to stop the motor,
as disabling the PWM output will have a small delay due to the motor controller
needing to detect the missing signal.

Note Set(0) is not a safe approach for CAN motor controllers, which may have closed
loop operation, non-% output set() calls, etc.

wpilibc/src/main/native/cpp/motorcontrol/PWMMotorController.cpp

@@ -31,7 +31,7 @@ void PWMMotorController::Disable() {
 }
 
 void PWMMotorController::StopMotor() {
-  Disable();
+  Set(0);
 }
 
 std::string PWMMotorController::GetDescription() const {