driver: i2c: npcx: clear status register by setting a single bit.

Since all fields' type in SMBST is R/W1C and RO, setting a single bit to
clear a specified event is a more suitable solution. Or we might clear
the other pending bits that occurred at the same moment unexpectedly.

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