igb: XDP xmit back fix error code

The igb XDP xmit back function should only return
defined error codes.

Fixes: 9cbc948b ("igb: add XDP support")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Sven Auhagen <sven.auhagen@voleatech.de>
Tested-by: Sandeep Penigalapati <sandeep.penigalapati@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

drivers/net/ethernet/intel/igb/igb_main.c

@@ -2910,7 +2910,7 @@ static int igb_xdp_xmit_back(struct igb_adapter *adapter, struct xdp_buff *xdp)
 	 */
 	tx_ring = adapter->xdp_prog ? igb_xdp_tx_queue_mapping(adapter) : NULL;
 	if (unlikely(!tx_ring))
-		return -ENXIO;
+		return IGB_XDP_CONSUMED;
 
 	nq = txring_txq(tx_ring);
 	__netif_tx_lock(nq, cpu);