serial: 8250: Auto CTS control by HW if AFE enabled

According to DesignWare 8250 spec, if auto flow control
mode is enabled, a change in CTS does not cause an interrupt,
so sw-assisted CTS flow control mode will not work properly.

There reported an GPS firmware download failure issue, and we
verified the root cause is, the default sw-assisted CTS flow
control mode can not work properly since no interrupt when got
CTS signal.

This patch is to enable auto CTS mode by defaut if CRTSCTS
is enable for DesignWare 8250 controller.

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