USB: usbip: fix stub_dev hub disconnect

If a hub is disconnected that has device(s) that's attached to the usbip layer
the disconnect function might fail because it tries to release the port
on an already disconnected hub.

Fixes: 6080cd0e

 ("staging: usbip: claim ports used by shared devices")
Signed-off-by: Jonas Blixt <jonas.blixt@actia.se>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://lore.kernel.org/r/20230615092810.1215490-1-jonas.blixt@actia.se
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>