TradHeli: ch6 tuning of ext gyro to use control_in

This could also be used to control the other Heli Ch7 functions (direct drive tail rotor control).  This should be tidied up.

Credit: Jolyon Saunders

ArduCopter/ArduCopter.pde

@@ -2232,7 +2232,7 @@ static void tuning(){
 
 #if FRAME_CONFIG == HELI_FRAME
     case CH6_HELI_EXTERNAL_GYRO:
-        motors.ch7_pwm_setpoint(tuning_value);
+        motors.ch7_pwm_setpoint(g.rc_6.control_in);
         break;
 #endif