cpu/atmega_common: Fix race in periph_timer
cpu/atmega_common: Fix race in periph_timer

As AVR is an 8 bit platform, special care needs to be taken when accessing 16
bit registers in an atomic fashion. This commit as just this care.

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