fixed clock running too fast on ATmega168p/ATmega328p

The original Code selects a PWM mode instead of CTC, which causes the system clock to run too fast.
WGM13 is not changed anywhere in this file. Updating it to include WGM13 has nothing to do with the bugfix and could better go in its own commit, since there are also a lot of other controller files that were not updated to include WGM13.