staging: comedi: dmm32at: use 8255 module for Digital I/O subdevice

The Dimond-MM-32-AT board uses an internal 82C55-type digital I/O circuit to
provide the 24 digital I/O lines. The only quirk is the need to set the page
selection bits in the control register to select page 1 addresses.

Instead of duplicating the 8255 code, provide an (*io) callback and use the
8255 module to support this subdevice.

This also removes the need for the private data in this driver.

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>