Timer simulation fixes and improvements

Several fixes after refactoring: 
- wrong brackets led to wrong top in case of WGM PWM
- although mode was correctly set in avr_timer_write, it was erased in avr_timer_reconfigure

And improvements for PWM mode
- there was no clear/set on TOV in case OCRnX was in clear/set mode
- PWM for OCRnB was not called if top was OCRnA