cpu/stm32: fix kconfig model usb clock
cpu/stm32: fix kconfig model usb clock

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