I have to admit I just tested it with the old kernel and it also works. So in the end including the prism module into the kernel wasn't necessary at all. The problem at first just was that I exactly followed the Debian wiki instructions and therefore did not rename the firmware!

I'm glad to here you got it working like I said in my post. If the file is different or not present, this could be part of the problem."
All you need to do is rename the file to the one I have showing.
I glad to hear it was an easy fix.