start simplifying the parsing of USB control URBs

Change-Id: Ie527b89f6be2d100dfd8bc7c96c2f2ac7efa3efc
Reviewed-on: https://code.wireshark.org/review/3735
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>