AP_Tuning: use rc() method to get rc singleton

libraries/AP_Tuning/AP_Tuning.cpp

@@ -65,7 +65,7 @@ void AP_Tuning::check_selector_switch(void)
         // no selector switch enabled
         return;
     }
-    RC_Channel *selchan = RC_Channels::rc_channel(selector-1);
+    RC_Channel *selchan = rc().channel(selector-1);
     if (selchan == nullptr) {
         return;
     }
@@ -173,7 +173,7 @@ void AP_Tuning::check_input(uint8_t flightmode)
         return;
     }
     
-    RC_Channel *chan = RC_Channels::rc_channel(channel-1);
+    RC_Channel *chan = rc().channel(channel-1);
     if (chan == nullptr) {
         return;
     }