USB: keyspan: fix null-deref at disconnect and release

commit ff8a43c10f1440f07a5faca0c1556921259f7f76 upstream.

Make sure to fail properly if the device is not accepted during attach
in order to avoid null-pointer derefs (of missing interface private
data) at disconnect or release.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>