ITE: drivers/i2c: Enable I2C interrupt after completing configuration

Fix the flow of I2C enable interrupt. We should enable the interrupt
after I2C configuration is completed to avoid pending interrupts and
cause errors irq.

Test port:
i2c_ite_it8xxx2: i2c0
i2c_ite_enhance: i2c4
Test:
tests\drivers\i2c\i2c_api --> pass

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