Staging: usbip: fix multiple interfaces

The stub_probe function instantiates an stub_dev object  for all
interfaces. Wich causes a problem.  The stub_dev object belongs to their
own interfaces. This patch creates the sdev object at the first
stub_probe call, the other calls associate the interfaces to this.

Signed-off-by: Endre Kollar <taxy443@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>