mei: fix NULL dereferencing during FW initiated disconnection

In the case when disconnection is initiated from the FW
the driver is flushing items from the write control list while
iterating over it:

mei_irq_write_handler()
    list_for_each_entry_safe(ctrl_wr_list)         <-- outer loop
         mei_cl_irq_disconnect_rsp()
             mei_cl_set_disconnected()
                 mei_io_list_flush(ctrl_wr_list)   <-- destorying list

We move the list flushing to the completion routine.

Cc: <stable@vger.kernel.org> #4.2+
Signed-off-by: Alexander Usyskin <alexander.usyskin@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/mei/client.c

@@ -1766,6 +1766,10 @@ void mei_cl_complete(struct mei_cl *cl, struct mei_cl_cb *cb)
 		if (waitqueue_active(&cl->wait))
 			wake_up(&cl->wait);
 
+		break;
+	case MEI_FOP_DISCONNECT_RSP:
+		mei_io_cb_free(cb);
+		mei_cl_set_disconnected(cl);
 		break;
 	default:
 		BUG_ON(0);