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

commit e1bfdbc7 upstream.

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

Fixes: bdbd0a7f

 ("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>