The DMA wait queue makes the code very complicated:
For RX, the @->stop_rx hook does not really stop the RX;
For TX, the @->stop_tx hook does not really stop the TX.
The above make the imx_shutdown has to wait the RX/TX DMA to be finished.
In order to make code more simple, this patch removes the DMA wait queue.
By calling the dmaengine_terminate_all, this patch makes the RX stops
immediately after we call the @->stop_rx hook, so does the TX.