serial: sh-sci: protect invalidating RXDMA on shutdown

The to-be-fixed commit removed locking when invalidating the DMA RX
descriptors on shutdown. It overlooked that there is still a rx_timer
running which may still access the protected data. So, re-add the
locking.

Reported-by: Dirk Behme <dirk.behme@de.bosch.com>
Closes: https://lore.kernel.org/r/ee6c9e16-9f29-450e-81da-4a8dceaa8fc7@de.bosch.com
Fixes: 2c4ee23 ("serial: sh-sci: Postpone DMA release when falling back to PIO")
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20240506114016.30498-7-wsa+renesas@sang-engineering.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/sh-sci.c

@@ -1270,9 +1270,14 @@ static void sci_dma_rx_chan_invalidate(struct sci_port *s)
 static void sci_dma_rx_release(struct sci_port *s)
 {
 	struct dma_chan *chan = s->chan_rx_saved;
+	struct uart_port *port = &s->port;
+	unsigned long flags;
 
+	uart_port_lock_irqsave(port, &flags);
 	s->chan_rx_saved = NULL;
 	sci_dma_rx_chan_invalidate(s);
+	uart_port_unlock_irqrestore(port, flags);
+
 	dmaengine_terminate_sync(chan);
 	dma_free_coherent(chan->device->dev, s->buf_len_rx * 2, s->rx_buf[0],
 			  sg_dma_address(&s->sg_rx[0]));