powerpc/kdump: Use chip->shutdown to disable IRQs

I saw this in a kdump kernel:

IOMMU table initialized, virtual merging enabled
Interrupt 155954 (real) is invalid, disabling it.
Interrupt 155953 (real) is invalid, disabling it.

ie we took some spurious interrupts. default_machine_crash_shutdown tries
to disable all interrupt sources but uses chip->disable which maps to
the default action of:

static void default_disable(unsigned int irq)
{
}

If we use chip->shutdown, then we actually mask the IRQ:

static void default_shutdown(unsigned int irq)
{
        struct irq_desc *desc = irq_to_desc(irq);

        desc->chip->mask(irq);
        desc->status |= IRQ_MASKED;
}

Not sure why we don't implement a ->disable action for xics.c, or why
default_disable doesn't mask the interrupt.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>