staging: comedi: adl_pci9118: remove 'ai16bits' from private data

This member of the private data is only set when the analog input
subdevice maxdata is 0xffff. Just check the s->maxdata and remove
the private data 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>