net: stmmac: fix cbs configuration

Sending again, because forgot to include net-dev.

The QoS IP does not accept AVB capabilities to default/queue 0, this way we
guarantee 75% bandwidth for AVB. This patch assures that only queues >= 1
gets CBS confgured. Additional info was also added to stmmac.txt.

Reported-by: Niklas Cassel <niklas.cassel@axis.com>
Signed-off-by: Joao Pinto <jpinto@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/stmmac.txt

@@ -103,6 +103,8 @@ Optional properties:
 		- Choose one of these modes:
 			- snps,dcb-algorithm: TX queue will be working in DCB
 			- snps,avb-algorithm: TX queue will be working in AVB
+			  [Attention] Queue 0 is reserved for legacy traffic
+			  and so no AVB is available in this queue.
 		- Configure Credit Base Shaper (if AVB Mode selected):
 			- snps,send_slope: enable Low Power Interface
 			- snps,idle_slope: unlock on WoL

drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

@@ -1880,7 +1880,8 @@ static void stmmac_configure_cbs(struct stmmac_priv *priv)
 	u32 mode_to_use;
 	u32 queue;
 
-	for (queue = 0; queue < tx_queues_count; queue++) {
+	/* queue 0 is reserved for legacy traffic */
+	for (queue = 1; queue < tx_queues_count; queue++) {
 		mode_to_use = priv->plat->tx_queues_cfg[queue].mode_to_use;
 		if (mode_to_use == MTL_QUEUE_DCB)
 			continue;