serial: sh-sci: Replace direct stop_rx/stop_tx calls with port ops in sci_shutdown()

Replace direct calls to sci_stop_rx() and sci_stop_tx() with port ops
callbacks in sci_shutdown(). This enables the RSCI driver, which reuses
the SCI core but implements its own stop_rx and stop_tx logic, to reuse
sci_shutdown() without duplicating code.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20250630202323.279809-4-prabhakar.mahadev-lad.rj@bp.renesas.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>