We now generalize the USB communiction and abandon the 'req_ctx'
structure inherited from openpcd. Instead we use the libosmocore 'msgb'
structure to handle incoming and outgoing USB tranfers. We also use
linuxlist-based msgb-queues for each endpoint.
This makes sure that we'll re-enumerate on the USB, as a CPU reset
apparently doesn't automatically release the pull-up and notify the hub
that we were gone?
