driver: i2c: npcx: don't print the err log when SMBST is zero

In the I2C ISR, it prints the error message when SMBST is set to an
unexpected value. However, we found a spurious interrupt which caused
the SoC to enter the ISR with SMBST=0. Because the spurious interrupt
will not break the I2C operation, this commit limits the error log to
print if SMBST is not equivalent to 0 to prevent a false alert.

Signed-off-by: Andrew McRae <amcrae@google.com>
Signed-off-by: Jun Lin <CHLin56@nuvoton.com>