x86/PCI: fix unused variable warning in amd_bus.c

Fix this warning:

  arch/x86/pci/amd_bus.c: In function 'early_fill_mp_bus_info':
  arch/x86/pci/amd_bus.c:56:6: warning: unused variable 'j' [-Wunused-variable]

introduced by commit d28e5ac2a07e ("x86/PCI: dynamically allocate
pci_root_info for native host bridge drivers").

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>