Copter: fixed motor test build on heli

pwm percent makes no sense

ArduCopter/motor_test.cpp

@@ -38,11 +38,13 @@ void Copter::motor_test_output()
 
             case MOTOR_TEST_THROTTLE_PERCENT:
                 // sanity check motor_test_throttle value
+#if FRAME_CONFIG != HELI_FRAME
                 if (motor_test_throttle_value <= 100) {
                     int16_t pwm_min = motors.get_pwm_output_min();
                     int16_t pwm_max = motors.get_pwm_output_max();
                     pwm = pwm_min + (pwm_max - pwm_min) * (float)motor_test_throttle_value/100.0f;
                 }
+#endif
                 break;
 
             case MOTOR_TEST_THROTTLE_PWM: