staging: comedi: cb_pcidda: cleanup DADATA define

Rename the define used for the D/A Data register so that is has
namespace with this driver. Change the define so that it takes
the channel as a parameter to calculate the correct register
offset.

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>