serial: pch_uart: fix tty-kref leak in dma-rx path

commit 19b85cfb upstream.

Fix tty_kref leak when tty_buffer_request room fails in dma-rx path.

Note that the tty ref isn't really needed anymore, but as the leak has
always been there, fixing it before removing should makes it easier to
backport the fix.
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/tty/serial/pch_uart.c

@@ -552,11 +552,12 @@ static int dma_push_rx(struct eg20t_port *priv, int size)
 		dev_warn(port->dev, "Rx overrun: dropping %u bytes\n",
 			 size - room);
 	if (!room)
-		return room;
+		goto out;
 
 	tty_insert_flip_string(tty, sg_virt(&priv->sg_rx), size);
 
 	port->icount.rx += room;
+out:
 	tty_kref_put(tty);
 
 	return room;