it8xxx2: intc: ensure IER disabling to become effective

We put disabling SOC interrupt enable register (IER) sequence in
between disable and enable core's global interrupt to prevent race
condition.
After core interrupt enable instruction has been executed, the new
configuration of IER has not yet been fully processed due to
asynchronization between core and SOC's source clock.

If SOC interrupt is fired under the above condition, we will get
IRQ number 0 in ISR due to IER disabling taken effect.

Signed-off-by: Dino Li <Dino.Li@ite.com.tw>