serial: 8250_dw: fix 'cts-override'

We are dealing with CTS, not DSR here (we dealt with DSR a few lines
above), so set appropriate bits.

Reported-by: Kevin Cernekee <cernekee@chromium.org>
Signed-off-by: Dmitry Torokhov <dtor@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>