PCI/ERR: Simplify by using pci_upstream_bridge()

PCI/ERR: Simplify by using pci_upstream_bridge()

Use pci_upstream_bridge() in place of dev->bus->self.  No functional change
intended.

Link: https://lore.kernel.org/r/20201121001036.8560-6-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>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>