create audio-specific part of the usb conversation
store the major spec version there

Change-Id: I55564cbca7fb9faa307f1a684858d2e5d2393fc9
Reviewed-on: https://code.wireshark.org/review/3581
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>