staging: comedi: cb_pcimdda: remove attached_to_8255 from private data

The attached_to_8255 variable in the private data is used as a flag
to indicate that the 8255 subdevice has been initialized. The call
to subdev_8255_cleanup only requires that the dev->subdevices pointer
is valid. Change the test in the detach function and remove the
attached_to_8255 variable.

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>