tests/periph_uart: fix periph type cast
tests/periph_uart: fix periph type cast

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