usbip: keep usbip_device sockfd state in sync with tcp_socket

Keep usbip_device sockfd state in sync with tcp_socket. When tcp_socket
is reset to null, reset sockfd to -1 to keep it in sync.

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>