AP_EFI: prevent flood of Lutan serial port

send request at max 5Hz

libraries/AP_EFI/AP_EFI_Serial_Lutan.cpp

@@ -91,7 +91,7 @@ void AP_EFI_Serial_Lutan::update()
             pkt_nbytes = 0;
         }
     }
-    if (n == 0 || now - last_request_ms > 200) {
+    if (now - last_request_ms > 200) {
         last_request_ms = now;
         port->discard_input();
         send_request();