Further NXT USB cleanups

[laurensvalk]

There are a number of follow ups for pybricks/pybricks-micropython#436 that still need to be done:

• Remove some of the blocking waits in the init/reset.
• There is some remaining logic from nxos for breaking packets into smaller chunks. I think this is only used for EP0 now, so we could clean this up accordingly.
• Consider dropping : from the serial number so it is the same as the original firmware.
• There are numerous TODOs in the nxt_usb file.
• Need to figure out why the status is not ready in pbdrv_usb_tx_response. See the REVISIT note there.
• We dropped support for the two separate RX banks here. Maybe we should keep it.
• This one is nice. TODO: Make this a little nicer. Not quite sure how.
• Implement this comment.

[laurensvalk]

It looks like the NXT USB has an ADC pin attached to it that we could use to detect plugging in? I don't suppose we really need it, but just posting it here for the record.