Allow non-sudo usage of tc420

[schwabix-1311]

Using the tc420 command line tool on various Linux systems, I always needed to run it via sudo. Now I'm embedding the class TC420 in a larger project, where this is no option.

On stackoverflow I found the hint to have the udev rule set MODE="0666" instead of "0664". I'm far from understanding udev, but this change seems to help.