staging: comedi: dmm32at: rename DMM32AT_AUXDOUT

For aesthetics, rename this define used for the Auxillary Digital Output
register. This register is currently not used in the driver. Document the
bits of the register for completeness.

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>