it8xxx2/espi: protect clear OBF request

At default, IBF (input buffer full) interrupt status will be cleared
after reading keyboard data input register (KBHIDIR) in ISR.
For request to clear OBF (output buffer full), we need to enable clear
mode. In the mode, IBF status cannot be cleared by reading KBHIDIR
register. It means that if AP output data to 60h/64h port during the
mode enable, IBF interrupt will keep triggering until the watchdog is
reset. This patch addresses this issue.

Signed-off-by: Dino Li <Dino.Li@ite.com.tw>