i8259: Reorder intack in pic_read_irq

As we want to move the IRQ update to pic_intack, ordering matters: the
slave ack must be executed before the master ack to avoid missing
further pending slave IRQs.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>