802.11: Enhance Supported Operating Classes

* Use expert_info to display warning about length (Remove proto_tree_add_text)

Change-Id: I36ce6b6fc1138c53621da4913e23f9f1ceaaca50
Reviewed-on: https://code.wireshark.org/review/5369
Reviewed-by: Michael Mann <mmann78@netscape.net>