cpu/efm32/timer_series2: fix timer_start() interaction with pm_layered
cpu/efm32/timer_series2: fix timer_start() interaction with pm_layered

The timer hasn't been enabled yet, if the STATUS isn't RUNNING, yet ...

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