staging: comedi: ni_660x: tidy up the misc. constants

Remove enum ni_660x_constants and just #define the value.

Move all the constant #defines so they are in one place and rename
them so they are more conesistent.

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>