Cycles: change default Principled Hair BSDF back to Chiang

The Huang model has issues with fireflies in various scenes, and the
defualt choice is somewhat arbitrary anyway as they have both pros and
cons. For now be conservative and keep the default the same as before.

source/blender/nodes/shader/nodes/node_shader_bsdf_hair_principled.cc

@@ -138,7 +138,7 @@ static void node_shader_init_hair_principled(bNodeTree * /*ntree*/, bNode *node)
 {
   NodeShaderHairPrincipled *data = MEM_cnew<NodeShaderHairPrincipled>(__func__);
 
-  data->model = SHD_PRINCIPLED_HAIR_HUANG;
+  data->model = SHD_PRINCIPLED_HAIR_CHIANG;
   data->parametrization = SHD_PRINCIPLED_HAIR_REFLECTANCE;
 
   node->storage = data;