staging: comedi: ni_670x: fix dereference of an invalid pointer

The attach if this driver can fail before the subdevices are
allocated. If the attach fails the comedi core will call the
detach routine to allow the driver to do it's cleanup.

We need to make sure that the subdevice allocation was successful
before trying to dereference subdevice[0] to free the allocated
range table list.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>