staging: comedi: dmm32at: remove unused members of the private data

The 'data' and 'ai_inuse' members in the private data are not used in the
driver. Remove them.

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