PCI/CXL: Hide SBR from reset_methods if masked by CXL

Per CXL r3.1, sec 8.1.5.2, the Secondary Bus Reset (SBR) bit in the Bridge
Control register of a CXL port has no effect unless the "Unmask SBR" bit in
the Port Control Extensions Register is set.

After b1956e2d0713 ("PCI/CXL: Fail bus reset if upstream CXL Port has SBR
masked"), Linux checks the "Unmask SBR" bit in pci_reset_bus_function().
But when probe==true, it previously returned 0, incorrectly indicating that
SBR is a viable reset method for the device.

As a result, "bus" is listed in the device's "reset_method" attribute even
though the hardware is incapable of performing it. If a user writes "bus"
to "reset_method" or triggers a reset that falls back to SBR, the operation
fails with "write error: Inappropriate ioctl for device".

If the link is operating in CXL mode (pcie_is_cxl()), return -ENOTTY
immediately unless "Unmask SBR" is set, regardless of the probe argument.
This ensures that "bus" is not advertised in "reset_methods" when the
hardware prevents it, improving clarity for users and aligning the sysfs
capability report with actual hardware behavior.

Signed-off-by: Vidya Sagar <vidyas@nvidia.com>
[bhelgaas: commit log, use pcie_is_cxl()]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Link: https://patch.msgid.link/20260225133801.30231-1-vidyas@nvidia.com

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