platform/x86/intel/vsec: Create wrapper to walk PCI config space

platform/x86/intel/vsec: Create wrapper to walk PCI config space

Combine three PCI config space walkers — intel_vsec_walk_dvsec(),
intel_vsec_walk_vsec(), and intel_vsec_walk_header() — into a new wrapper
function, intel_vsec_feature_walk().  This refactoring simplifies the probe
logic and lays the groundwork for future patches that will loop over these
calls. No functional changes.

Signed-off-by: David E. Box <david.e.box@linux.intel.com>
Link: https://lore.kernel.org/r/20250703022832.1302928-4-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>