MIPS: PCI: Fix pcibios_scan_bus() NULL check code path

If pci_scan_root_bus() fails (ie returns NULL) pcibios_scan_bus() must
return immediately since the struct pci_bus pointer it returns is not valid
and cannot be used.

Move code checking the pci_scan_root_bus() return value to reinstate proper
pcibios_scanbus() error path behaviour.

Fixes: 88555b48

 ("MIPS: PCI: Support for CONFIG_PCI_DOMAINS_GENERIC")
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Paul Burton <paul.burton@imgtec.com>