greybus: hd: add flag argument to cport_enable callback

Add a flag argument to the host-device cport_enable callback that can be
used to provide hints about the connection being created (e.g.
connection priority).

Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>