staging: comedi: pcl816: add support for digital subdevices

The PCL-816 boards have 16 digital inputs and 16 digtial outputs. Add the
subdevice support for these channels.

Allocate additional subdevice space to the analog outputs. This code is
not currently in the driver so mark the subdevice as COMEDI_SUBD_UNUSED.

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>