Fixes in avr_timer_reconfigure after refactoring

Several fixes after fcce7868a2fe2ef028b7f6c1741a12380b82a9cc:
- Remove zeroing of mode in which erased mode
  seleced in avr_timer_write
- Fix wrong brackets in ?: in avr_timer_wgm_pwm mode
Enhance emulation of PWM mode

- Clear/set output pin on TOP when OCRnX is in clear/set mode,
  previously it was changed only on TCNT=OCRnX
- Call PWM irq for OCRnB even when TOP is OCRnA