PCI: Accept AtomicOps already enabled by the hypervisor

pci_enable_atomic_ops_to_root() currently fails when no Root Port is
visible. That is common in passthrough guests (ESXi, Hyper-V): the Endpoint
is assigned to the VM, but the Root Port above it is not visible in the
guest topology.

In those setups the hypervisor may already have enabled AtomicOp Requester
Enable on the device. If PCI_EXP_DEVCTL2_ATOMIC_REQ is set, treat AtomicOps
as already enabled and return success instead of failing the Root Port
walk.

After 1ae8c4ce1570 ("PCI: Enable AtomicOps only if Root Port supports
them"), pci_enable_atomic_ops_to_root() always returns failure if the Root
Port is not visible, so drivers don't use atomics when they could.  On
systems where the Root Port is not visible but *does* support AtomicOps,
this is a regression: prior to 1ae8c4ce1570, it enabled AtomicOps in the
endpoint and returned success.

Fixes: 1ae8c4ce1570 ("PCI: Enable AtomicOps only if Root Port supports them")
Signed-off-by: Nikola Prica <nikola.prica@amd.com>
[bhelgaas: commit log, code comment]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Tested-by: Gerd Bayer <gbayer@linux.ibm.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Gerd Bayer <gbayer@linux.ibm.com>
Link: https://patch.msgid.link/20260921111903.978687-1-nikprica@amd.com

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