net: dsa: allow switches to work without tagging

In case switch port tagging is disabled (voluntarily, or the switch just
does not support it), allow us to continue using the defined set of
dsa_device_ops in net/dsa/slave.c.

We introduce dsa_protocol_is_tagged() to check whether we need to
override skb->protocol and go through the DSA-specifif packet_type
function, or if we just go on and receive the SKB through the normal
path.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -1784,10 +1784,10 @@ void dev_net_set(struct net_device *dev, struct net *net)
 static inline bool netdev_uses_dsa(struct net_device *dev)
 {
 #ifdef CONFIG_NET_DSA
-	return dev->dsa_ptr != NULL;
-#else
-	return false;
+	if (dev->dsa_ptr != NULL)
+		return dsa_uses_tagged_protocol(dev->dsa_ptr);
 #endif
+	return false;
 }
 
 /**