Bluetooth: Mesh: Add return value for opcode callback

```
3.7.3.4 Message error procedure
When receiving a message that is not understood by an element, it shall
ignore the message.
Note: A message can be falsely identified as a valid message, passing
the NetMIC and TransMIC authentication using a known network key and
application key even though that message was sent using different keys.
The decryption of that message using the wrong keys would result in a
message that is not understood by the element. The probability of such a
situation occurring is small but not insignificant.
A message that is not understood includes messages that have one or more
of the following conditions:
• The application opcode is unknown by the receiving element.
• The access message size for the application opcode is incorrect.
• The application parameters contain values that are currently
Prohibited.
Note: An element that sends an acknowledged message that is not
understood by a peer node will not receive any response message.
```

Signed-off-by: Michał Narajowski <[email protected]>

Author: Michał Narajowski

include/bluetooth/mesh/access.h

@@ -175,10 +175,12 @@ struct bt_mesh_model_op {
 	 *  @param ctx   Message context for the message.
 	 *  @param buf   Message buffer containing the message payload, not
 	 *               including the opcode.
+	 *
+	 *  @return Zero on success or (negative) error code otherwise.
 	 */
-	void (*const func)(struct bt_mesh_model *model,
-			   struct bt_mesh_msg_ctx *ctx,
-			   struct net_buf_simple *buf);
+	int (*const func)(struct bt_mesh_model *model,
+			  struct bt_mesh_msg_ctx *ctx,
+			  struct net_buf_simple *buf);
 };
 
 #define BT_MESH_MODEL_OP_1(b0) (b0)

subsys/bluetooth/mesh/access.c

@@ -670,7 +670,7 @@ void bt_mesh_model_recv(struct bt_mesh_net_rx *rx, struct net_buf_simple *buf)
 		 * receive the message.
 		 */
 		net_buf_simple_save(buf, &state);
-		op->func(model, &rx->ctx, buf);
+		(void)op->func(model, &rx->ctx, buf);
 		net_buf_simple_restore(buf, &state);
 	}
 }