Feature request: Full-Speed PRE handling

[desowin]

Currently the capture speed is fixed for a capture session duration. This is always fine for High-Speed and Low-Speed captures and mostly fine for Full-Speed captures. There is one corner case when it is not fine for Full-Speed captures: when capture the link between host and hub while there is Low-Speed device connected.

Due to very strict timings, the speed change must be handled by FPGA. The Low-Speed packet captured on Full-Speed link should probably be indicated with new flag. Universal Serial Bus Specification Revision 2.0 section 8.6.5 Low-speed Transactions describes what is sent at Full-Speed and what at Low-Speed.

To obtain best Wireshark experience, the data would have to be written to pcapng format. I can handle the pcapng related work if the bitstream handles the PRE processing and sets low-speed flag on packets.