PCI/DPC: Allow DPC on all Downstream Ports when OS controls AER

PCIe r7.0, sec 6.2.11, "Implementation Note: Determination of DPC Control",
recommends that "... operating systems always link control of DPC to the
control of Advanced Error Reporting."

Any PCIe device may advertise AER, but only Root Ports and Root Complex
Event Collectors can generate AER interrupts, so the AER driver only binds
to RPs and RCECs.

Any Root Port or Switch Downstream Port may advertise Downstream Port
Containment (DPC), but previously the DPC driver was limited to devices the
AER driver could bind to, i.e., only RPs that advertised AER.

Since any Port with DPC can generate DPC interrupts, allow the DPC driver
to bind to such a Port as long as the OS controls AER, regardless of
whether the AER driver binds to it.

Signed-off-by: Darshit Shah <darnshah@amazon.de>
[bhelgaas: commit log, reorder || operands to simplify patch]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20251211164257.81655-1-darnshah@amazon.de

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