drivers/at86rf2xx: fix NETOPT_RANDOM
drivers/at86rf2xx: fix NETOPT_RANDOM

- fix wrong return value
- fix wrong size parameter
    - note, the behavior is a bit relaxed in that it allows
      NETOPT_RANDOM with different size than `sizeof(uint32_t)`

kaspar030/RIOT,kYc0o/RIOT,jasonatran/RIOT,jasonatran/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,kYc0o/RIOT,kYc0o/RIOT,miri64/RIOT,RIOT-OS/RIOT,ant9000/RIOT,ant9000/RIOT,ant9000/RIOT,miri64/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,ant9000/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,kYc0o/RIOT,kYc0o/RIOT,miri64/RIOT,ant9000/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,miri64/RIOT,jasonatran/RIOT,miri64/RIOT