avr_timer: changed avr_timer_write_ocr as part of larger avr_timer refactor.

- changed avr_timer_write_ocr code to take pointer of relevant
  	comparator structure rather than its parent timer.

- added _timer_get_comp_ocr which the modified avr_timer_write_ocr
	function uses.

	modified:   simavr/sim/avr_timer.c
	modified:   simavr/sim/avr_timer.h