staging: comedi: ni_660x: rename the CamelCase enum NI_660x_Register and labels

As prefered by the CodingStyle, rename this CamelCase enum and its labels.

Also, cleanup the ni_gpct_to_660x_register() helper function. Just return the
ni_660x_register for each ni_gpct_register and remove the unnecessary break
statements after the return statements.
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>