staging: comedi: daqboard2000: redo DAC status macros and fix busy

Rename the macros defining values for the DAC status register to avoid
CamelCase, and to make it clear which register they are associated with.
Refactor the macros defining the regular DAC channel "busy" bits into a
single macro that takes the DAC channel number as a parameter.

Add a macro to define the offset of the read-only DAC status register.
It is the same offset as the DAC control register, which is write-only.

The code in `daqboard2000_ao_eoc()` that checks the status for
completion of the DAC conversion looks wrong.  The register has a "busy"
bit for each channel, but the existing code only works for channels 0
and 1.  The driver only supports two DAC channels at the moment, so the
bug is currently harmless, but fix it so we can support four DAC
channels on some board models.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>