PCI/ACPI: Update link to PCI firmware specification

The previous link to the PCI firmware specification in the comments
for drivers/pci/pci-acpi.c no longer works.  Update the comment
to a current link to this specification.

Link: https://lore.kernel.org/r/20220722174754.27921-1-mario.limonciello@amd.com


Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>