staging: comedi: adv_pci1724: remove enum sync_output_trigger_contents

This enum is not used in the driver. Remove it.

For aesthetics, rename the register define and move the comment that
any value will trigger the sync output.

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>