USB: make hub driver's release more robust

This revised patch (as893c) improves the method used by the hub driver
to release its private data structure.  The current code is non-robust,
relying on a memory region not getting reused by another driver after
it has been freed.  The patch adds a reference count to the structure,
resolving the question of when to release it.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>