PCI/ERR: Add pci_walk_bridge() to pcie_do_recovery()

Consolidate subordinate bus checks with pci_walk_bus() into
pci_walk_bridge() for walking below potentially AER affected bridges.

Link: https://lore.kernel.org/r/20201121001036.8560-10-sean.v.kelley@intel.com


Tested-by: Jonathan Cameron <Jonathan.Cameron@huawei.com> # non-native/no RCEC
Signed-off-by: Sean V Kelley <sean.v.kelley@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>