platform/x86/intel/vsec: Restore BAR fallback for header walk

platform/x86/intel/vsec: Restore BAR fallback for header walk

The base_addr refactor changed intel_vsec_walk_header() to pass
info->base_addr as the discovery-table base address. For the PCI VSEC
driver this info comes from driver_data, but exported callers may provide
their own static headers and leave base_addr unset.

For xe, this made the discovery-table base address zero instead of the BAR
selected by header->tbir, preventing PMT endpoints from being created.

Restore the previous behavior for the header-walk path by falling back to
pci_resource_start(pdev, header->tbir) when base_addr is not specified.
Keep explicit base_addr override behavior unchanged.

This preserves the refactor structure while fixing the functional
regression in manual-header users.

Fixes: 904b333fc51c ("platform/x86/intel/vsec: Refactor base_addr handling")
Assisted-by: Claude:claude-sonnet-4-6
Signed-off-by: David E. Box <david.e.box@linux.intel.com>
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Link: https://patch.msgid.link/20260529183150.129744-1-david.e.box@linux.intel.com
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>