Sub: fixed startup order of setting mavlink system ID

Set sysid from param before you init the serial port so you always output the intended sysid. Ported the same fix as c01a7718c1

ArduSub/system.cpp

@@ -32,6 +32,9 @@ void Sub::init_ardupilot()
 
     BoardConfig.init();
 
+    // identify ourselves correctly with the ground station
+    mavlink_system.sysid = g.sysid_this_mav;
+    
     // initialise serial port
     serial_manager.init();
 
@@ -60,9 +63,6 @@ void Sub::init_ardupilot()
         gcs_chan[i].setup_uart(serial_manager, AP_SerialManager::SerialProtocol_MAVLink, i);
     }
 
-    // identify ourselves correctly with the ground station
-    mavlink_system.sysid = g.sysid_this_mav;
-
 #if LOGGING_ENABLED == ENABLED
     log_init();
 #endif