powerpc: Rework Axon MSI setup so we can avoid freeing the irq_host

If we do the call to irq_of_parse_and_map() first, then we don't
need to worry about freeing the irq_host.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>