cpu/nrf51: fix NDEBUG compile problem
cpu/nrf51: fix NDEBUG compile problem


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