serial:8250_dw: do not alter CTS and DCTS since AFE is enabled

Since the serial core is informed that this device is doing
automatic flow control, it is not necessary to alter CTS and
DCTS of the MSR as the core no longer attempts stop the port
in uart_handle_cts_change().

Signed-off-by: Huiquan Zhong <huiquan.zhong@intel.com>
Signed-off-by: Qipeng Zha <qipeng.zha@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>