staging: comedi: adq12b: remove digital_state from private data

Use the subdevice 'state' variable instead of carrying the state of
the output channels in the private data.

Use comedi_dio_update_state() to handle the boilerplate code to update
the subdevice s->state.

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>