powerpc: Rework ipic_init() so we can avoid freeing the irq_host

If we do the call to of_address_to_resource() first, then we don't
need to worry about freeing the irq_host (which the code doesn't do
currently anyway).

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