serial: sh-sci: Stop TX DMA workqueue during port shutdown

The transmit DMA workqueue is never stopped, hence the work function may
be called after the port has been shut down.

Fix this race condition by cancelling queued work, if any, before DMA
release.  Don't initialize the work if DMA initialization failed, as it
won't be used anyway.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/sh-sci.c

@@ -1293,6 +1293,7 @@ static void sci_tx_dma_release(struct sci_port *s)
 {
 	struct dma_chan *chan = s->chan_tx_saved;
 
+	cancel_work_sync(&s->work_tx);
 	s->chan_tx_saved = s->chan_tx = NULL;
 	s->cookie_tx = -EINVAL;
 	dmaengine_terminate_all(chan);
@@ -1548,10 +1549,9 @@ static void sci_request_dma(struct uart_port *port)
 				__func__, UART_XMIT_SIZE,
 				port->state->xmit.buf, &s->tx_dma_addr);
 
+			INIT_WORK(&s->work_tx, work_fn_tx);
 			s->chan_tx_saved = s->chan_tx = chan;
 		}
-
-		INIT_WORK(&s->work_tx, work_fn_tx);
 	}
 
 	chan = sci_request_dma_chan(port, DMA_DEV_TO_MEM);