APM: fixed elevon mixing

this was broken in 2.61

ArduPlane/Attitude.pde

@@ -398,8 +398,10 @@ static void set_servos(void)
         
 
         // push out the PWM values
+        if (g.mix_mode == 0) {
             g.channel_roll.calc_pwm();
             g.channel_pitch.calc_pwm();
+        }
         g.channel_throttle.calc_pwm();
         g.channel_rudder.calc_pwm();
     }