timer: Avoid infinite cycle timer on TCNT write

timer: Avoid infinite cycle timer on TCNT write

In some situations, it was possible to enter an infinite cycle timer
loop. In avr_timer_tcnt_write, the tov cycle timer registration is not
protected and can register a timer with p->tov_cycles == 0.

The following atmega1280 program demonstrates this issue:

int main() {

	TCCR1A = 0;
	TCCR1B = (1<<WGM12) | (1<<ICES1);
	OCR1B = OCR1A = 960;

	/* Start */
	TCNT1 = 0;
	TCCR1B |= (1<<CS11);

	/* Stop */
	TCCR1B &= ~(1<<CS11);
	TIFR1 |= (1<<OCF1A) | (1<<OCF1B);

	/* Start */
	TCNT1 = 0; /**< Registers timer with tov_cycles == 0. */
	TCCR1B |= (1<<CS11);

	while (1) ;
	return 0;
}