staging: comedi: pcl812: tidy up the remaining register defines

The PCL812_CNTENABLE define is not used in this driver. Remove it.

For aesthetics, rename the software trigger register.

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>