drivers/counter/it8xxx2: fix loss timer interrupt potential risk

1.correct timer register control flow
2.select timer interrupt rising edge trigger, instead of default
level trigger

Stress test: top timer fires interrupt every 300ms for 18 hours.
The result is that we don't lose any interrupts.

Signed-off-by: Ruibin Chang <Ruibin.Chang@ite.com.tw>