When Rx in-band wakeup is enabled, set RTS to true in mtk8250_shutdown()
so the connected device can still send message and trigger IRQ when the
system is suspended.

Fixes: 18c9d4a3 ("serial: When UART is suspended, set RTS to false")
Cc: stable <stable@kernel.org>
Signed-off-by: Pin-yen Lin <treapking@chromium.org>
Link: https://lore.kernel.org/r/20240424130619.2924456-1-treapking@chromium.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>