staging: comedi: addi_apci_3501: start defining i/o map

Create, and use, defines for the analog output and digital i/o
registers.

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>