greybus: greybus_protocols: svc: Add missing comment

All request/responses either have a structure representing them or a
comment saying the request/response payload doesn't exist.

The comment was missing for route create response message, add it.

Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>