AP_ICEngine: use rc() method to get rc singleton

libraries/AP_ICEngine/AP_ICEngine.cpp

@@ -124,9 +124,10 @@ void AP_ICEngine::update(void)
     }
 
     uint16_t cvalue = 1500;
-    if (start_chan != 0) {
+    RC_Channel *c = rc().channel(start_chan-1);
+    if (c != nullptr) {
         // get starter control channel
-        cvalue = RC_Channels::get_radio_in(start_chan-1);
+        cvalue = c->get_radio_in();
     }
 
     bool should_run = false;
@@ -269,9 +270,10 @@ bool AP_ICEngine::engine_control(float start_control, float cold_start, float he
         state = ICE_OFF;
         return true;
     }
-    if (start_chan != 0) {
+    RC_Channel *c = rc().channel(start_chan-1);
+    if (c != nullptr) {
         // get starter control channel
-        if (RC_Channels::get_radio_in(start_chan-1) <= 1300) {
+        if (c->get_radio_in() <= 1300) {
             gcs().send_text(MAV_SEVERITY_INFO, "Engine: start control disabled");
             return false;
         }