[POWERPC] Fix possible NULL deref in ppc32 PCI

The 32-bit PCI code tests if "bus" is non-NULL after calling
pci_scan_bus_parented() in one place but not another before
dereferencing it.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>