PCI: brcmstb: Prevent config space access when link is down

When the link is down, config accesses to downstream devices cause CPU
aborts.  Allow config accesses only when the link is up.

As the following scenario shows, this check is racy and cannot completely
avoid CPU aborts, but it makes them less likely:

  pci_generic_config_read
    addr = brcm_pcie_map_conf            # bus->ops->map_bus()
      brcm_pcie_link_up                  # returns "true"; link is up
    <link goes down>
    *val = readb(addr)                   # link is now down
    <CPU abort>

Note that config space accesses to the Root Port are not affected by link
status.

[bhelgaas: commit log, use PCIE_ECAM_REG() instead of magic 0xfff masks;
note that pci_generic_config_read32() masks low two bits already]
Link: https://lore.kernel.org/r/20220725151258.42574-4-jim2101024@gmail.com


Signed-off-by: Jim Quinlan <jim2101024@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Tested-by: Florian Fainelli <f.fainelli@gmail.com>