merge dissect_linux_usb_pseudo_header_ext() and
try_dissect_linux_usb_pseudo_header_ext()

Change-Id: Ica9e3089cc6bfde9b52158ef5e161dfcddeb716a
Reviewed-on: https://code.wireshark.org/review/3898
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>