relay USB control messages without payload to protocol-specific dissectors

relay USB control messages without payload to protocol-specific dissectors

at the moment, we don't forward such messages and therefore see lots
of generic USB control messages that could be dissected further

even if there's no data, a protocol-specific dissector may still set
the columns based on conversation info

Change-Id: If3fc0f0ce3bdec1f91b7e3cadc3affd56b8c8969
Reviewed-on: https://code.wireshark.org/review/7584
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>