cpu/stm32l0: fix clk control register reset
cpu/stm32l0: fix clk control register reset

on stm32l011, RCC_CR_CSSON is not defined

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