sfc: Use write-combining to reduce TX latency

Based on work by Neil Turton <nturton@solarflare.com> and
Kieran Mansley <kmansley@solarflare.com>.

The BIU has now been verified to handle 3- and 4-dword writes within a
single 128-bit register correctly.  This means we can enable write-
combining and only insert write barriers between writes to distinct
registers.

This has been observed to save about 0.5 us when pushing a TX
descriptor to an empty TX queue.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>

drivers/net/sfc/efx.c

@@ -1104,8 +1104,8 @@ static int efx_init_io(struct efx_nic *efx)
 		rc = -EIO;
 		goto fail3;
 	}
-	efx->membase = ioremap_nocache(efx->membase_phys,
-				       efx->type->mem_map_size);
+	efx->membase = ioremap_wc(efx->membase_phys,
+				  efx->type->mem_map_size);
 	if (!efx->membase) {
 		netif_err(efx, probe, efx->net_dev,
 			  "could not map memory BAR at %llx+%x\n",