set_latency_timer: allow 0ms duration

[srwalter]

The datasheet for the FT2232H says:

"Latency Timer. This is really a feature of the driver and is used
to as a timeout to flush short packets of data back to the PC. The
default is 16ms, but it can be altered between 0ms and 255ms. At 0ms
latency you get a packet transfer on every high speed microframe."

I have observed that using 0 results in significantly improved throughput with small packets on the FT2232H.

[newAM]

Created a new release with this fix, 0.32.3

[Farmadupe]

The quoted text from the datasheet for the FT2232H directly conflicts with the following wording in the official API docs:

In the FT8U232AM and FT8U245AM devices, the receive buffer timeout that is used to flush remaining
data from the receive buffer was fixed at 16 ms. In all other FTDI devices, this timeout is programmable
and can be set at 1 ms intervals between 2ms and 255 ms. This allows the device to be better optimized
for protocols requiring faster response times from short data packets.

This forces us to have to decide which official documentation is correct. I suggest the driver docs should overrule the FT2232H datasheet, as at best the datasheet applies to a single device, but the driver applies to every device. Assuming that the API docs are correct, the removal of the above assertion actually creates an unsoundness issue as values below 2 are explicitly forbidden.

Possible solutions may be:

• Reintroduce range checks (would suggest full assertions rather than debug assertions, as perf is probably not an issue)
• Mark this function as unsafe
• Add an unsafe_set_latency_timer function to allow <2ms timeouts.