serial: sh-sci: Stop transfers in sci_shutdown()

Make sure the transmitter and receiver are stopped when shutting down
the port, and related interrupts are disabled.

Without this:
  - New input data may be received into the RX FIFO, possibly
    triggering a new RX DMA completion,
  - Transfers will still be enabled on a subsequent startup of the UART,
    before the UART's FIFOs have been reset, causing reading of stale
    data.

Inspired by a patch in the BSP by Koji Matsuoka
<koji.matsuoka.xm@renesas.com>.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/sh-sci.c

@@ -1989,6 +1989,7 @@ static void sci_shutdown(struct uart_port *port)
 {
 	struct sci_port *s = to_sci_port(port);
 	unsigned long flags;
+	u16 scr;
 
 	dev_dbg(port->dev, "%s(%d)\n", __func__, port->line);
 
@@ -1998,6 +1999,9 @@ static void sci_shutdown(struct uart_port *port)
 	spin_lock_irqsave(&port->lock, flags);
 	sci_stop_rx(port);
 	sci_stop_tx(port);
+	/* Stop RX and TX, disable related interrupts, keep clock source */
+	scr = serial_port_in(port, SCSCR);
+	serial_port_out(port, SCSCR, scr & (SCSCR_CKE1 | SCSCR_CKE0));
 	spin_unlock_irqrestore(&port->lock, flags);
 
 #ifdef CONFIG_SERIAL_SH_SCI_DMA