ixgb: Remove useless DMA-32 fallback configuration

As stated in [1], dma_set_mask() with a 64-bit mask never fails if
dev->dma_mask is non-NULL.
So, if it fails, the 32 bits case will also fail for the same reason.

So, if dma_set_mask_and_coherent() succeeds, 'pci_using_dac' is known to be
1.

Simplify code and remove some dead code accordingly.

[1]: https://lkml.org/lkml/2021/6/7/398Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Alexander Lobakin <alexandr.lobakin@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

drivers/net/ethernet/intel/ixgb/ixgb_main.c

@@ -361,7 +361,6 @@ ixgb_probe(struct pci_dev *pdev, const struct pci_device_id *ent)
 	struct net_device *netdev = NULL;
 	struct ixgb_adapter *adapter;
 	static int cards_found = 0;
-	int pci_using_dac;
 	u8 addr[ETH_ALEN];
 	int i;
 	int err;
@@ -370,17 +369,11 @@ ixgb_probe(struct pci_dev *pdev, const struct pci_device_id *ent)
 	if (err)
 		return err;
 
-	pci_using_dac = 0;
 	err = dma_set_mask_and_coherent(&pdev->dev, DMA_BIT_MASK(64));
-	if (!err) {
-		pci_using_dac = 1;
-	} else {
-		err = dma_set_mask_and_coherent(&pdev->dev, DMA_BIT_MASK(32));
 	if (err) {
 		pr_err("No usable DMA configuration, aborting\n");
 		goto err_dma_mask;
 	}
-	}
 
 	err = pci_request_regions(pdev, ixgb_driver_name);
 	if (err)
@@ -444,10 +437,8 @@ ixgb_probe(struct pci_dev *pdev, const struct pci_device_id *ent)
 			   NETIF_F_HW_VLAN_CTAG_FILTER;
 	netdev->hw_features |= NETIF_F_RXCSUM;
 
-	if (pci_using_dac) {
 	netdev->features |= NETIF_F_HIGHDMA;
 	netdev->vlan_features |= NETIF_F_HIGHDMA;
-	}
 
 	/* MTU range: 68 - 16114 */
 	netdev->min_mtu = ETH_MIN_MTU;