Revert "Revert "create audio-specific part of the usb conversation""

This reverts commit 22e10bea0cbd2b29d6e7b82a3ead3910d71988a2.

Now that the fuzz failures are fixed, we can re-enable the
audio-specific extensions.

Change-Id: Id788e10aaa3dbf289df364f7da9ef01e24ba47f9
Reviewed-on: https://code.wireshark.org/review/3728
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>