Fix async cleanup to hold a kref count on the URBs

See the comments in the code and in the patch. URBs are freed by
the USB core code that calls the URB completion routine.

OXT-30

Signed-off-by: Ross Philipson <ross.philipson@gmail.com>
Merge pull request #5 from rossphilipson/urb_misery

Fix async cleanup to hold a kref count on the URBs