Your damp patch has developed because the water is being fed through the wall by the cable.
What you need to do is go outside where the cable enters the wall and run the cable down the wall past the hole. Hammer a cable clip into the wall below the hole You need to create a loop and return the cable up the wall to the hole. This creates a low point where the water will then naturally flow to the low point, insted of being fed through the wall.
You can fill the hole with silicone, but I doubt you will need it.