staging: comedi: adl_pci9118: refactor 'ai_maxdata' in boardinfo

The boards supported by this driver either have 12-bit or 16-bit analog
inputs. For aesthetics, replace the 'ai_maxdata' member in the boardinfo
with a bit-field flag, 'ai_is_16bit', and refactor the subdevice init to
use the new member.

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>