staging: comedi: dt9812: rename the private data struct

The comedi private data (dev->private) for this driver is stored
in a 'struct comedi_dt9812' that is allocated during the comedi
(*attach).

For aesthetic reasons, rename this struct to help make its use
clearer.

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>