net: stmmac: No need to calculate speed divider when offload is disabled

commit be27b896 ("net: stmmac: replace priv->speed with
the portTransmitRate from the tc-cbs parameters") introduced
a problem. When deleting, it prompts "Invalid portTransmitRate
0 (idleSlope - sendSlope)" and exits. Add judgment on cbs.enable.
Only when offload is enabled, speed divider needs to be calculated.

Fixes: be27b896 ("net: stmmac: replace priv->speed with the portTransmitRate from the tc-cbs parameters")
Signed-off-by: Xiaolei Wang <xiaolei.wang@windriver.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240617013922.1035854-1-xiaolei.wang@windriver.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

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

@@ -358,24 +358,28 @@ static int tc_setup_cbs(struct stmmac_priv *priv,
 
 	port_transmit_rate_kbps = qopt->idleslope - qopt->sendslope;
 
-	/* Port Transmit Rate and Speed Divider */
-	switch (div_s64(port_transmit_rate_kbps, 1000)) {
-	case SPEED_10000:
-	case SPEED_5000:
-		ptr = 32;
-		break;
-	case SPEED_2500:
-	case SPEED_1000:
-		ptr = 8;
-		break;
-	case SPEED_100:
-		ptr = 4;
-		break;
-	default:
-		netdev_err(priv->dev,
-			   "Invalid portTransmitRate %lld (idleSlope - sendSlope)\n",
-			   port_transmit_rate_kbps);
-		return -EINVAL;
+	if (qopt->enable) {
+		/* Port Transmit Rate and Speed Divider */
+		switch (div_s64(port_transmit_rate_kbps, 1000)) {
+		case SPEED_10000:
+		case SPEED_5000:
+			ptr = 32;
+			break;
+		case SPEED_2500:
+		case SPEED_1000:
+			ptr = 8;
+			break;
+		case SPEED_100:
+			ptr = 4;
+			break;
+		default:
+			netdev_err(priv->dev,
+				   "Invalid portTransmitRate %lld (idleSlope - sendSlope)\n",
+				   port_transmit_rate_kbps);
+			return -EINVAL;
+		}
+	} else {
+		ptr = 0;
 	}
 
 	mode_to_use = priv->plat->tx_queues_cfg[queue].mode_to_use;