drivers/sensor/ite/tach_ite_it8xxx2: don't clear both tach status at once

When two tach use simultaneously, I find that the both tach status will
be cleared at once, which causes one of tach not to get rpm speed.
So I change it to clear status only one tach at a time.

This issue is reported by Huaqin:
https://partnerissuetracker.corp.google.com/issues/404067663?pli=1



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