driver: i2c: npcx: clear FIFO status before a transaction starts

When an I2C transaction completes, the driver should clear all the FIFO
status. Otherwise, it has the chance to break the operation of the next
transaction. This commit sets the CLR_FIFO bit in the SMBFIF_CTS
register to clear all the FIFO status at the beginning of an I2C
transaction.

Signed-off-by: Jun Lin <CHLin56@nuvoton.com>