arch-atmega/pwm.c: fix tim_res_ns of OC0B

Signed-off-by: Aurelio Colosimo <aurelio@aureliocolosimo.it>
arch-atmega/pwm.c: fix set_ and get_period timer1 (period is ICR1 + 1)

Signed-off-by: Aurelio Colosimo <aurelio@aureliocolosimo.it>