staging: comedi: adv_pci1724: tidy up the register I/O map

For aesthetics, convert the enum board_registers into defines and rename
them so they have namespace associated with 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>