drivers/ethos: fix build without ethos_stdio
drivers/ethos: fix build without ethos_stdio

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