cpu/efm32: fix build with !LETIMER
cpu/efm32: fix build with !LETIMER

Not all SoCs have an LETIMER.
Here the compilation will will fail because LETIMER related
symbols are not defined.

Fix the build of timer.c for those.

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