USB: keyspan: fix null-deref at probe

commit b5122236bba8d7ef62153da5b55cc65d0944c61e upstream.

Fix null-pointer dereference during probe if the interface-status
completion handler is called before the individual ports have been set
up.

Fixes: f79b2d0f ("USB: keyspan: fix NULL-pointer dereferences and
memory leaks")
Reported-by: Richard <richjunk@pacbell.net>
Tested-by: Richard <richjunk@pacbell.net>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>