sfc: simplify DMA mask setting

Christoph says[1] that dma_set_mask_and_coherent() is smart enough to
 truncate the mask itself if it's too long.  So we can get rid of our
 "lop off one bit and retry" loop in efx_init_io().

[1]: https://www.spinics.net/lists/netdev/msg677266.html

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/sfc/efx_common.c

@@ -1086,17 +1086,7 @@ int efx_init_io(struct efx_nic *efx, int bar, dma_addr_t dma_mask,
 
 	pci_set_master(pci_dev);
 
-	/* Set the PCI DMA mask.  Try all possibilities from our
-	 * genuine mask down to 32 bits, because some architectures
-	 * (e.g. x86_64 with iommu_sac_force set) will allow 40 bit
-	 * masks event though they reject 46 bit masks.
-	 */
-	while (dma_mask > 0x7fffffffUL) {
-		rc = dma_set_mask_and_coherent(&pci_dev->dev, dma_mask);
-		if (rc == 0)
-			break;
-		dma_mask >>= 1;
-	}
+	rc = dma_set_mask_and_coherent(&pci_dev->dev, dma_mask);
 	if (rc) {
 		netif_err(efx, probe, efx->net_dev,
 			  "could not find a suitable DMA mask\n");