serial: 8520_mtk: Set RTS on shutdown for Rx in-band wakeup

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>

drivers/tty/serial/8250/8250_mtk.c

@@ -209,15 +209,19 @@ static int mtk8250_startup(struct uart_port *port)
 
 static void mtk8250_shutdown(struct uart_port *port)
 {
-#ifdef CONFIG_SERIAL_8250_DMA
 	struct uart_8250_port *up = up_to_u8250p(port);
 	struct mtk8250_data *data = port->private_data;
+	int irq = data->rx_wakeup_irq;
 
+#ifdef CONFIG_SERIAL_8250_DMA
 	if (up->dma)
 		data->rx_status = DMA_RX_SHUTDOWN;
 #endif
 
-	return serial8250_do_shutdown(port);
+	serial8250_do_shutdown(port);
+
+	if (irq >= 0)
+		serial8250_do_set_mctrl(&up->port, TIOCM_RTS);
 }
 
 static void mtk8250_disable_intrs(struct uart_8250_port *up, int mask)