lan743x: 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.

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>
Link: https://lore.kernel.org/r/ef548716606f257939df9738a801f15b6edf2568.1641743405.git.christophe.jaillet@wanadoo.frSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/microchip/lan743x_main.c

@@ -1739,13 +1739,10 @@ static int lan743x_tx_ring_init(struct lan743x_tx *tx)
 	}
 	if (dma_set_mask_and_coherent(&tx->adapter->pdev->dev,
 				      DMA_BIT_MASK(64))) {
-		if (dma_set_mask_and_coherent(&tx->adapter->pdev->dev,
-					      DMA_BIT_MASK(32))) {
-			dev_warn(&tx->adapter->pdev->dev,
-				 "lan743x_: No suitable DMA available\n");
-			ret = -ENOMEM;
-			goto cleanup;
-		}
+		dev_warn(&tx->adapter->pdev->dev,
+			 "lan743x_: No suitable DMA available\n");
+		ret = -ENOMEM;
+		goto cleanup;
 	}
 	ring_allocation_size = ALIGN(tx->ring_size *
 				     sizeof(struct lan743x_tx_descriptor),
@@ -2284,13 +2281,10 @@ static int lan743x_rx_ring_init(struct lan743x_rx *rx)
 	}
 	if (dma_set_mask_and_coherent(&rx->adapter->pdev->dev,
 				      DMA_BIT_MASK(64))) {
-		if (dma_set_mask_and_coherent(&rx->adapter->pdev->dev,
-					      DMA_BIT_MASK(32))) {
-			dev_warn(&rx->adapter->pdev->dev,
-				 "lan743x_: No suitable DMA available\n");
-			ret = -ENOMEM;
-			goto cleanup;
-		}
+		dev_warn(&rx->adapter->pdev->dev,
+			 "lan743x_: No suitable DMA available\n");
+		ret = -ENOMEM;
+		goto cleanup;
 	}
 	ring_allocation_size = ALIGN(rx->ring_size *
 				     sizeof(struct lan743x_rx_descriptor),