Rover: log mode in dataflash callback

this ensures every log has the initial mode

APMrover2/Log.cpp

@@ -431,6 +431,7 @@ void Rover::Log_Write_Vehicle_Startup_Messages()
 {
     // only 200(?) bytes are guaranteed by DataFlash
     Log_Write_Startup(TYPE_GROUNDSTART_MSG);
+    DataFlash.Log_Write_Mode(control_mode);
 }
 
 // start a new log