One tester reported that disconnecting in the middle of a scatterlist
operation would give a lot of confusing messages.  This should improve
that situation a bunch:  most of the messages were pointless.  It also
resolves some related FIXMEs for both queue cleanup modes (cancelation,
and after transport fault).

  - now only urbs in flight have non-null urb->dev, so we can avoid
    trying to unlink unsubmitted or (common) already-completed urbs.

  - it's ok to get -EBUSY unlink status, it just means the urb's
    already being given back (though since it's in flight, it's
    not an error to try unlinking).

Also, the relevant diagnostics now use dev_*() driver model calls,
making the kernel strings smaller and the output more useful.