net: ethernet: ti: am65-cpsw: Fix NULL dereference on XDP_TX

If number of TX queues are set to 1 we get a NULL pointer
dereference during XDP_TX.

~# ethtool -L eth0 tx 1
~# ./xdp-trafficgen udp -A <ipv6-src> -a <ipv6-dst> eth0 -t 2
Transmitting on eth0 (ifindex 2)
[  241.135257] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000030

Fix this by using actual TX queues instead of max TX queues
when picking the TX channel in am65_cpsw_ndo_xdp_xmit().

Fixes: 8acacc40 ("net: ethernet: ti: am65-cpsw: Add minimal XDP support")
Signed-off-by: Roger Quadros <rogerq@kernel.org>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Acked-by: Julien Panis <jpanis@baylibre.com>
Reviewed-by: MD Danish Anwar <danishanwar@ti.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

drivers/net/ethernet/ti/am65-cpsw-nuss.c

@@ -1924,12 +1924,13 @@ static int am65_cpsw_ndo_bpf(struct net_device *ndev, struct netdev_bpf *bpf)
 static int am65_cpsw_ndo_xdp_xmit(struct net_device *ndev, int n,
 				  struct xdp_frame **frames, u32 flags)
 {
+	struct am65_cpsw_common *common = am65_ndev_to_common(ndev);
 	struct am65_cpsw_tx_chn *tx_chn;
 	struct netdev_queue *netif_txq;
 	int cpu = smp_processor_id();
 	int i, nxmit = 0;
 
-	tx_chn = &am65_ndev_to_common(ndev)->tx_chns[cpu % AM65_CPSW_MAX_TX_QUEUES];
+	tx_chn = &common->tx_chns[cpu % common->tx_ch_num];
 	netif_txq = netdev_get_tx_queue(ndev, tx_chn->id);
 
 	__netif_tx_lock(netif_txq, cpu);