avr_timer: refactor avr_timer_write and avr_timer_reconfigure

avr_timer_reconfigure: calculations moved back in call chain to
 avr_timer_write.

avr_timer_write_ocr: use data processed in avr_timer_write.

avr_timer_init: changed to fully trap writes going to as2,
 clock select bits and waveform generation mode bits.

	modified:   ../../simavr/sim/avr_timer.c
	modified:   ../../simavr/sim/avr_timer.h