staging: comedi: pcl730: clean up the register map documentation

This driver supports a number of simple digital i/o boards with
slight variations in the register map. Clean up the comment about
the register map to better document the driver.

Remove the #define's used for the registers since thet don't apply
to all the board variations supported by the driver.

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>