stm32/periph/uart: set flow control bits before enabling uart
stm32/periph/uart: set flow control bits before enabling uart

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