RC_Channel: make get_control_mid const

libraries/RC_Channel/RC_Channel.cpp

@@ -230,7 +230,7 @@ RC_Channel::calc_pwm(void)
   used for thr_mid in copter
  */
 int16_t
-RC_Channel::get_control_mid() {
+RC_Channel::get_control_mid() const {
     if (_type == RC_CHANNEL_TYPE_RANGE) {
         int16_t r_in = (radio_min.get()+radio_max.get())/2;
 

libraries/RC_Channel/RC_Channel.h

@@ -62,7 +62,7 @@ public:
     uint8_t     get_ch_out(void) const { return _ch_out; };
 
     // get the center stick position expressed as a control_in value
-    int16_t     get_control_mid();
+    int16_t     get_control_mid() const;
 
     // read input from APM_RC - create a control_in value
     void        set_pwm(int16_t pwm);