usbip: fix error handling in stub_probe()

If usb_hub_claim_port() fails, no resources are deallocated and
if stub_add_files() fails, port is not released.

The patch fixes these issues and rearranges error handling code.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Acked-by: Valentina Manea <valentina.manea.m@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>