staging: comedi: aio_aio12_8: fix ai (*insn_read)

According to the user manual, analog input bipolar data is 2's
complement and unipolar is straight binry. Use the core helpers
to munge the data appropriately.

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>