staging: comedi: dt282x: remove the 'opt_*' enum

This enum is used to index the 'options' values that are passed
by the user attaches to the driver. These options are identified
in the comedi comment block.

For aesthetics, remove the enum and just open code the index values.

Refactor the Analog Input subdevice init so that the it->options[4],
indicating differential inputs, does not have to be checked multiple
times.

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>