staging: comedi: cb_pcidas: tidy up DAC control/status register

Rename this register and its bit defines so they have namespace
associated with the driver. Use the BIT macro to define the bits.

For aesthetics, rename the associated member of the private data.

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>