USB: consider the device class when we try to select the next dissector

at the moment, we only use the interface class, even in situations where
it's not applicable (e.g. control request that's not addressed to an
interface or an endpoint) - this is a hack that works because some
dissectors register for interface class "unknown", I'm trying to make
this workaround obsolete

Change-Id: I6bf4253a2f8bf5a75ea1a08c3e52d36f80abd7ab
Reviewed-on: https://code.wireshark.org/review/5215
Petri-Dish: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Sean Stalley <seanstalley@gmail.com>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>