ITE: drivers/i2c: Don't spam NACK error messages

Printing of NACK messages should be set to LOG_DBG to avoid spamming.
When we scan whether there is a target device through I2C, if we use
LOG_ERR, it will frequently print out NACK messages. So it is set to
LOG_DBG in the case of NACK.

Signed-off-by: Tim Lin <tim2.lin@ite.corp-partner.google.com>