sh: fix Oops in the serial SCI driver

Fix an Oops, triggering, if the DMA buffer allocation for the Rx channel in
sh-sci.c fails.
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>

drivers/serial/sh-sci.c

@@ -1004,8 +1004,9 @@ static void sci_rx_dma_release(struct sci_port *s, bool enable_pio)
 	s->chan_rx = NULL;
 	s->cookie_rx[0] = s->cookie_rx[1] = -EINVAL;
 	dma_release_channel(chan);
-	dma_free_coherent(port->dev, s->buf_len_rx * 2,
-			  sg_virt(&s->sg_rx[0]), sg_dma_address(&s->sg_rx[0]));
+	if (sg_dma_address(&s->sg_rx[0]))
+		dma_free_coherent(port->dev, s->buf_len_rx * 2,
+				  sg_virt(&s->sg_rx[0]), sg_dma_address(&s->sg_rx[0]));
 	if (enable_pio)
 		sci_start_rx(port);
 }