greybus: uart: Implement dtr_rts callback.

Dtr_dts allows the tty layer to set the flow control lines
to the correct state during open and close.

Note that locking for newctrl is missing throughout the
driver and will be addressed on a separate patch.

Suggested-and-reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Axel Haslam <ahaslam@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>