staging: comedi: dt3000: the dt3002 board does not have analog outputs

According to the boardinfo, the dt3002 board does not have analog outputs.
The rest of the board have two 12-bit analog output channels.

Replace the 'dachan' and 'dabits' members of the boardinfo with a bit-
field flag 'has_ao'. Use the new member to conditionally initialize
the analog output subdevice.

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>