Plane: ignore control mode changes when in throttle failsafe

this prevents the receiver causing a temporary mode change

ArduPlane/control_modes.pde

@@ -10,6 +10,12 @@ static void read_control_switch()
     // If we get this value we do not want to change modes.
     if(switchPosition == 255) return;
 
+    if (ch3_failsafe) {
+        // when we are in ch3_failsafe mode then RC input is not
+        // working, and we need to ignore the mode switch channel
+        return;
+    }
+
     // we look for changes in the switch position. If the
     // RST_SWITCH_CH parameter is set, then it is a switch that can be
     // used to force re-reading of the control switch. This is useful