Bluetooth: Mesh: Don't use Generic DTT server in Light LC server

MeshMDLv1.0.1:
Unlike for other server models, for Light LC server it is not specified
to use Generic DTT server if Transition Time field is not present in the
Light LC Light OnOff Set message.

MeshMDLv1.1r10:
Explicitly stated to use Light LC State Machine timer value when
Transition Time field is not present in the Light LC Light OnOff Set
message.

Related to #11587

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>

subsys/bluetooth/mesh/light_ctrl_srv.c

@@ -850,6 +850,21 @@ static int handle_light_onoff_get(struct bt_mesh_model *model, struct bt_mesh_ms
 	return 0;
 }
 
+static bool transition_get(struct bt_mesh_model *model,
+			   struct bt_mesh_model_transition *transition,
+			   struct net_buf_simple *buf)
+{
+	/* Light LC Server uses state machine values instead of DTT if Transition Time field is not
+	 * present in the message.
+	 */
+	if (buf->len == 2) {
+		model_transition_buf_pull(buf, transition);
+		return true;
+	}
+
+	return false;
+}
+
 static int light_onoff_set(struct bt_mesh_light_ctrl_srv *srv, struct bt_mesh_msg_ctx *ctx,
 			   struct net_buf_simple *buf, bool ack)
 {
@@ -862,7 +877,7 @@ static int light_onoff_set(struct bt_mesh_light_ctrl_srv *srv, struct bt_mesh_ms
 	uint8_t tid = net_buf_simple_pull_u8(buf);
 
 	struct bt_mesh_model_transition transition;
-	bool has_trans = !!model_transition_get(srv->model, &transition, buf);
+	bool has_trans = transition_get(srv->model, &transition, buf);
 
 	enum bt_mesh_light_ctrl_srv_state prev_state = srv->state;