net: renesas: rswitch: Fix return value in error path of xmit

Fix return value in the error path of rswitch_start_xmit(). If TX
queues are full, this function should return NETDEV_TX_BUSY.

Fixes: 3590918b ("net: ethernet: renesas: Add support for "Ethernet Switch"")
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/r/20230529073817.1145208-1-yoshihiro.shimoda.uh@renesas.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/renesas/rswitch.c

@@ -1485,7 +1485,7 @@ static netdev_tx_t rswitch_start_xmit(struct sk_buff *skb, struct net_device *nd
 
 	if (rswitch_get_num_cur_queues(gq) >= gq->ring_size - 1) {
 		netif_stop_subqueue(ndev, 0);
-		return ret;
+		return NETDEV_TX_BUSY;
 	}
 
 	if (skb_put_padto(skb, ETH_ZLEN))