staging: comedi: ni_mio_common: remove unnecessary use of 'board->adbits'

For aesthetics, use the subdevice 'maxdata' instead of the board 'adbits'.

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>