staging: usbip: removed unnecessary for loop

This for loop is not needed, since STUB_BUSID_OTHER is defined as 0.
In Addition added a comment if STUB_BUSID_OTHER changes sometime.

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>