staging: comedi: pcl818: remove 'ai_maxdata' from boardinfo

All the board types have 12-bit analog inputs. Remove this data from
the boardinfo.

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>