AP_SerialManager: fixed AP_Periph GPS

the disable of RX/TX pins broke AP_Periph on boards that don't enable
the GCS, as they don't have the SERIALn parameters, so we disable the
UART pins of the GPS

libraries/AP_SerialManager/AP_SerialManager.cpp

@@ -450,9 +450,14 @@ void AP_SerialManager::init()
             set_options(i);
             switch (state[i].protocol) {
                 case SerialProtocol_None:
+#if HAL_GCS_ENABLED
                     // disable RX and TX pins in case they are shared
-                    // with another peripheral (eg. RCIN pin)
+                    // with another peripheral (eg. RCIN pin). We
+                    // don't do this if GCS is not enabled as in that
+                    // case we don't have serialmanager parameters and
+                    // this would prevent AP_Periph from using a GPS
                     uart->disable_rxtx();
+#endif
                     break;
                 case SerialProtocol_Console:
                 case SerialProtocol_MAVLink: