usb: usbip: userspace: remove the port state file when detaching port.

with the last detached port state file remaining, usbip reports error on
attaching. So clean up the state files on detaching.

Signed-off-by: Harvey Yang <harvey.huawei.yang@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>