get conversation info when we parse audio class-specific descriptors
reject the descriptor if the class is not audio

Change-Id: Ie07f0fb711884e63cbe86d9417570108edd6b5ab
Reviewed-on: https://code.wireshark.org/review/3578
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>