distinguish between different audio descriptor types and subtypes
dissect the AC interface header descriptor

Change-Id: I10f81e5cbf3837d958b255b5a3405ac2465b171f
Reviewed-on: https://code.wireshark.org/review/3580
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>