tests/usbus: fix NDEBUG compile problem
tests/usbus: fix NDEBUG compile problem

OTAkeys/RIOT,ant9000/RIOT,ant9000/RIOT,miri64/RIOT,jasonatran/RIOT,kYc0o/RIOT,kYc0o/RIOT,OTAkeys/RIOT,jasonatran/RIOT,authmillenon/RIOT,authmillenon/RIOT,authmillenon/RIOT,basilfx/RIOT,ant9000/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,ant9000/RIOT,kYc0o/RIOT,yogo1212/RIOT,authmillenon/RIOT,authmillenon/RIOT,kaspar030/RIOT,jasonatran/RIOT,kYc0o/RIOT,OlegHahm/RIOT,ant9000/RIOT,OlegHahm/RIOT,yogo1212/RIOT,basilfx/RIOT,basilfx/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,yogo1212/RIOT,basilfx/RIOT,kaspar030/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,kaspar030/RIOT,miri64/RIOT,miri64/RIOT,yogo1212/RIOT,yogo1212/RIOT,kYc0o/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,miri64/RIOT,miri64/RIOT,kaspar030/RIOT,basilfx/RIOT,yogo1212/RIOT