staging: comedi: ni_stc.h: tidy up DAC[01]_Direct_Data registers

Rename the CamelCase and convert the defines into a macro.

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>