setup_irq() aborts immediately if there's no handler for the IRQ in
question. So i8259_init() should set up its handlers before trying to
set up the cascade on IRQ 2.

With this and the patch I sent a few days ago to fix initrd on ppc32, my
Pegasos now runs the arch/powerpc kernel.
Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>