staging: comedi: cb_pcidda: remove private data variable 'digitalio'

The private data variable 'digitalio' is only used to hold the PCI
base address for the 8255 devices on the board. This value is then
passed to subdev_8255_init() and stored in it's own private data.
There is no need to keep the value in the private data for the board.

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>