serdev: ttyport: ignore carrier detect to avoid hangups

Serdev currently does not support hangups so make sure to set CLOCAL to
prevent loss of carrier from triggering one.

Note however that not all tty drivers honour CLOCAL.

Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>