added try_dissect_linux_usb_pseudo_header_ext helper function

Change-Id: I05b0adaef2b979432e293bc26e2a122a0d3551fc
Reviewed-on: https://code.wireshark.org/review/3848
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Anders Broman <a.broman58@gmail.com>