staging: comedi: rti800: tidy up ai two's complement support

The analog input on this board can return data as either two's
complement or straight binary data. The format is selected by a
jumper of the board and enabled by the user as option[4] when
attaching to the board.

Replace the adc_coding enum with a simple bool to indicate that
the data is returned in two's complement form.

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>