staging: comedi: das16: change boardinfo 'ai_nbits' to 'ai_maxdata'

The subdevice functions that used this boardinfo can use the subdevice
's->maxdata' value instead. Tidy up those functions.

Change the boardinfo so that the calculation is not needed during the
attach.

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>