USB: keyspan: fix null-deref at probe

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

Fixes: f79b2d0fe81e ("USB: keyspan: fix NULL-pointer dereferences and
memory leaks")
Reported-by: Richard <richjunk@pacbell.net>
Tested-by: Richard <richjunk@pacbell.net>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>