greybus: uart fix missing negation on DTR setting

The unset the DTR flag is missing "~"

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