Commit 47a8b0cc (Enable PCIe AER only after checking firmware
support) wants to walk the PCI bus in the remove path to disable
AER, and calls pci_walk_bus for downstream bridges.

Unfortunately, in the remove path, we remove devices and bridges
in a depth-first manner, starting with the furthest downstream
bridge and working our way backwards.

The furthest downstream bridges will not have a dev->subordinate,
and we hit a NULL deref in pci_walk_bus.

Check for dev->subordinate first before attempting to walk the
PCI hierarchy below us.
Acked-by: NAndrew Patterson <andrew.patterson@hp.com>
Signed-off-by: NAlex Chiang <achiang@hp.com>
Signed-off-by: NMatthew Wilcox <willy@linux.intel.com>