Added dissection of Device Capability Descriptors in CapResp Packets

CapResp Packets may contain Device Capability Descriptors.
This patch allows wireshark to show what descriptors are present,
& checks that their length fields are valid.

See section 6.3.3 of the MA USB spec for more information.

Change-Id: Ia120435bdffcced091b67e66e0ba8f1a750358db
Reviewed-on: https://code.wireshark.org/review/3330
Petri-Dish: Anders Broman <a.broman58@gmail.com>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>