staging: comedi: ni_labpc: refactor labpc_8255_mmio()

Refactor the 8255 support code in preperation for using the comedi_device
'mmio' member.

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>