staging: comedi: cb_pcimdas: add main connector digital input/output

Add subdevices for the 4 digital input and 4 digital output channels on
the main connector of the board.

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>