spapr_pci: use the common _FDT() helper

All other users in hw/ppc already consider an error when building
the FDT to be fatal, even on hotplug paths. There's no valid reason
for spapr_pci to behave differently. So let's used the common _FDT()
helper which terminates QEMU when libfdt fails.

Signed-off-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
