staging: comedi: addi_apci_3120: define the Add-On registers

Define the add-On registers and bits and remove the "magic" numbers in the
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>