staging: comedi: ni_tio: tidy up enum ni_660x_second_gate_select

These values are not used as an enum. For aesthetics, rename the
CamelCase values and convert them into defines.

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>