staging: usbip: simplified cleanup function

This patch simplified "stub_device_free" cleanup function:
 - changed return type to void, since the return value is
   not checked anywhere
 - kfree is NULL-safe, so removed if statement
 - deleted debug-message

Signed-off-by: Kurt Kanzenbach <ly80toro@cip.cs.fau.de>
Signed-off-by: Stefan Reif <ke42caxa@cip.cs.fau.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>