staging: comedi: cb_pcidas: change dac bit enums into defines

The 'bits' for the DAC_CSR register are currently defined as
enums. All the other registers use defines for the bit definitions.
Change the dac bit enums to defines to follow the other registers.

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>