simplify dissect_nonstandard_usb_setup_request()
create setup_tvb in the calling function

Change-Id: Id34635afbd25817b09535fc52d1ba396b2597e5a
Reviewed-on: https://code.wireshark.org/review/3900
Petri-Dish: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>