staging: comedi: ni_tio.h: rename the CamelCase enum ni_gpct_register labels

As prefered by the CodingStyle, rename all the CamelCase labels of the
enum ni_gpct_register.

Cleanup all the helper functions in ni_tio_internal.h that used the enum.

The 'counter_index' parameter to all the functions is the ni_gpct_device
'counter_index' which is initialized when the subdevices are setup. This
value is always < 4 so the default: BUG() cases can never happen. For
aesthetics, rename the 'counter_idx' to simply 'idx' and fix the type in
some of the helpers.

Also, 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>