PCI: Add Extended Tag + MRRS quirk for Xeon 6

PCI: Add Extended Tag + MRRS quirk for Xeon 6

When bifurcated to x2, Xeon 6 Root Port performance is sensitive to the
configuration of Extended Tags, Max Read Request Size (MRRS), and 10-Bit
Tag Requester (note: there is currently no 10-Bit Tag support in the
kernel). While those can be configured to the recommended values by FW,
kernel may decide to overwrite the initial values.

Add a quirk that disallows enabling Extended Tags and setting MRRS
larger than 128B for devices under Xeon 6 Root Ports if the Root Port
is bifurcated to x2. Use the host bridge's enable_device hook to
overwrite MRRS if it's set to >128B for the device to be enabled.

The earlier attempts to implement this quirk polluted PCI core code with
the checks necessary to support this quirk. Using the enable_device hook
keeps the quirk well-contained, away from the PCI core code.

Suggested-by: Lukas Wunner <lukas@wunner.de>
Link: https://cdrdv2.intel.com/v1/dl/getContent/837176
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
[bhelgaas: 2x -> x2, rename quirk]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Link: https://patch.msgid.link/20250610114802.7460-1-ilpo.jarvinen@linux.intel.com