AP_EFI: use new UARTDriver discard_input method

libraries/AP_EFI/AP_EFI_Serial_MS.cpp

@@ -44,11 +44,7 @@ void AP_EFI_Serial_MS::update()
     }
 
     if (port->available() == 0 || now - last_response_ms > 200) {
-        // clear the input buffer
-        uint32_t buffered_data_size = port->available();
-        for (uint32_t i = 0; i < buffered_data_size; i++) {
-            port->read();
-        }
+        port->discard_input();
         // Request an update from the realtime table (7).
         // The data we need start at offset 6 and ends at 129
         send_request(7, RT_FIRST_OFFSET, RT_LAST_OFFSET);