staging: comedi: pcl812: remove 'ai_maxdata' from boardinfo

Most of the board types supported by this driver have 12-bit analog
inputs. Two of them, the acl8216 and a826pg, have 16-bit analog inputs.

Remove the 'ai_maxdata' member from the boardinfo and replace it with
a bit-field flag 'has_16bit_ai'. Refactor pcl812_attach() to use this
new 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>