OpenFlow: fix dissection of async_config_prop structure

Bug: 10808
Change-Id: I5ef46cd6b85c415e23dfdbe7a56c8b332eaae5a3
Reviewed-on: https://code.wireshark.org/review/6078
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>
Petri-Dish: Pascal Quantin <pascal.quantin@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Anders Broman <a.broman58@gmail.com>