powerpc/axon_msi: Validate msi irq via chip_data

Instead of checking for rogue msi numbers via the irq_map host field
set the chip_data to h.host_data (which is the msic struct pointer)
at map and compare it in get_irq.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>