driver: i2c: npcx: reset i2c semaphore before start transaction.

This CL reset i2c event-completed semaphore before starting
transactions. Some interrupt events such as BUS_ERROR might change its
counter when i2c bus is idle. It causes that the driver cannot wait
for the event completed and return immediately.

Signed-off-by: Mulin Chao <mlchao@nuvoton.com>