USB: fix dissection of non standard USB setup requests

Otherwise captures like the one found in bug 8161 do not dissect properly anymore (sub dissector is called twice)

While we are at it, reorganize functions order to void a forward declaration

Change-Id: Id02ef835ba24981902a1702f976db099fa07b0fd
Reviewed-on: https://code.wireshark.org/review/3858
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>