igb: 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>
Tested-by: Gurucharan G <gurucharanx.g@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

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

@@ -3164,8 +3164,8 @@ static int igb_probe(struct pci_dev *pdev, const struct pci_device_id *ent)
 	s32 ret_val;
 	static int global_quad_port_a; /* global quad port a indication */
 	const struct e1000_info *ei = igb_info_tbl[ent->driver_data];
-	int err, pci_using_dac;
 	u8 part_str[E1000_PBANUM_LENGTH];
+	int err;
 
 	/* Catch broken hardware that put the wrong VF device ID in
 	 * the PCIe SR-IOV capability.
@@ -3180,17 +3180,11 @@ static int igb_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) {
-			dev_err(&pdev->dev,
-				"No usable DMA configuration, aborting\n");
-			goto err_dma;
-		}
+	if (err) {
+		dev_err(&pdev->dev,
+			"No usable DMA configuration, aborting\n");
+		goto err_dma;
 	}
 
 	err = pci_request_mem_regions(pdev, igb_driver_name);
@@ -3306,8 +3300,7 @@ static int igb_probe(struct pci_dev *pdev, const struct pci_device_id *ent)
 	if (hw->mac.type >= e1000_i350)
 		netdev->hw_features |= NETIF_F_NTUPLE;
 
-	if (pci_using_dac)
-		netdev->features |= NETIF_F_HIGHDMA;
+	netdev->features |= NETIF_F_HIGHDMA;
 
 	netdev->vlan_features |= netdev->features | NETIF_F_TSO_MANGLEID;
 	netdev->mpls_features |= NETIF_F_HW_CSUM;