staging: comedi: dt3000: refactor 'adbits' boardinfo

All the board supported by this driver have analog inputs. The input
resolution is either 12-bit or 16-bit.

Replace the 'adbits' member of the boardinfo with a bit-field flag
'ai_is_16bits' and just set if for the 16-bit boards.

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>