If there is no ongoing traffic, analyzer can retain buffered traffic until capture is stopped

[martinling]

Due to the way that buffers are managed internally, the gateware will not respond to a poll from the host until it has 512 bytes of capture data ready, to fill a full DATA packet.

On an active high or full speed bus, this is guaranteed to happen regularly, due to the constant flow of SOF packets.

On a low speed bus however, there are no SOF packets, and the entire enumeration process may add up to less than 512 bytes of capture, meaning that no packets at all are seen in Packetry while the capture is ongoing.

Workaround: stop the capture to see the missing data. Buffers are flushed to the host when the capture is stopped.

Solution: we should be willing to send a short packet to the host if a full packet has not been achieved after some timeout.