Fixed TrackerRMS TKT-004235 by checking nullity of data->input_dev in mxt_input_sync().

* Symptom: if update_fw and update_cfg, kernel panic occurs.
* Reproducibility: 10%
* Root Cause:
  - If update_fw, the T6 will send a CFG_ERR message periodically.
  - After that, update_cfg process begin, the mxt_update_cfg_store() will invoke mxt_free_input_device() and nullify data->input_dev.
  - The CFG_ERR message will trigger mxt_interrupt(), and mxt_input_sync() will be invoked by mxt_process_messages_t44(). And mxt_input_sync() references a NULL data->input_dev and kernel panic occurs.