spapr_pci: handle FDT creation errors with _FDT()

spapr_pci: handle FDT creation errors with _FDT()

libfdt failures when creating the FDT should cause QEMU to terminate.

Let's use the _FDT() macro which does just that instead of propagating
the error to the caller. spapr_populate_pci_child_dt() no longer needs
to return a value in this case.

Note that, on the way, this get rids of the following nonsensical lines:

    g_assert(!ret);
    if (ret) {

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