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>