802.11: Enhance Extenede Channel Switch Announcement

* Use expert info to display warning about launch (remove proto_tree_add_text)
* Remove a check about Unknown data (Not need)

Change-Id: I3ec1d0f5b955019cfe43141df5f712eb1446dce3
Reviewed-on: https://code.wireshark.org/review/5368
Reviewed-by: Michael Mann <mmann78@netscape.net>