staging: comedi: pcl818: remove 'sub_ai' from private data

This member of the private data is a duplicate of the comedi_device
'read_subdev' member. Remove the private data member and use the
comedi_device member instead.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>