serial: 8250_mtk: Fix register address for XON/XOFF character

The XON1/XOFF1 character registers are at offset 0xa0 and 0xa8
respectively, so we cannot use the definition in serial_port.h.

Fixes: bdbd0a7 ("serial: 8250-mtk: modify baudrate setting")
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20220427132328.228297-4-angelogioacchino.delregno@collabora.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: AngeloGioacchino Del Regno

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

@@ -57,6 +57,9 @@
 #define MTK_UART_FEATURE_SEL	39	/* Feature Selection register */
 #define MTK_UART_FEAT_NEWRMAP	BIT(0)	/* Use new register map */
 
+#define MTK_UART_XON1		40	/* I/O: Xon character 1 */
+#define MTK_UART_XOFF1		42	/* I/O: Xoff character 1 */
+
 #ifdef CONFIG_SERIAL_8250_DMA
 enum dma_rx_status {
 	DMA_RX_START = 0,
@@ -278,8 +281,8 @@ static void mtk8250_set_flow_ctrl(struct uart_8250_port *up, int mode)
 			(serial_in(up, MTK_UART_EFR) &
 			(~(MTK_UART_EFR_HW_FC | MTK_UART_EFR_SW_FC_MASK))));
 
-		serial_out(up, UART_XON1, START_CHAR(port->state->port.tty));
-		serial_out(up, UART_XOFF1, STOP_CHAR(port->state->port.tty));
+		serial_out(up, MTK_UART_XON1, START_CHAR(port->state->port.tty));
+		serial_out(up, MTK_UART_XOFF1, STOP_CHAR(port->state->port.tty));
 		serial_out(up, UART_LCR, lcr);
 		mtk8250_disable_intrs(up, MTK_UART_IER_CTSI|MTK_UART_IER_RTSI);
 		mtk8250_enable_intrs(up, MTK_UART_IER_XOFFI);