==================== mctp: Add MCTP-over-USB hardware transport binding
Add an implementation of the DMTF standard DSP0283, providing an MCTP channel over high-speed USB.
This is a fairly trivial first implementation, in that we only submit one tx and one rx URB at a time. We do accept multi-packet transfers, but do not yet generate them on transmit.
Of course, questions and comments are most welcome, particularly on the USB interfaces.