serial: 8250_dw: Allow hardware flow control to be used

In the most common use case, the Synopsys DW UART driver does not
set the set_termios callback function.  This prevents UPSTAT_AUTOCTS
from being set when the UART flag CRTSCTS is set.  As a result, the
driver will use software flow control as opposed to hardware flow
control.

To fix the problem, the set_termios callback function is set to the
DW specific function.  The logic to set UPSTAT_AUTOCTS is moved so
that any clock error will not affect setting the hardware flow
control.

Reviewed-by: Scott Branden <scott.branden@broadcom.com>
Reviewed-by: Ray Jui <ray.jui@broadcom.com>
Signed-off-by: Jason Uy <jason.uy@broadcom.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>