tests/periph_timer: Fix frequency for waspmote-pro
tests/periph_timer: Fix frequency for waspmote-pro

The waspmote-pro cannot run at 250 kHz, as no available divider can divide
its core frequency down to that. Instead, the board can be tested at its
core clock frequency.

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