mfd: asic3: Fix potential null pointer dereference

We previously assumed 'mem_sdio' could be null but it is
dereferenced in ioremap(). Add a check to avoid a potential
null pointer dereference error.
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/mfd/asic3.c

@@ -899,13 +899,15 @@ static int __init asic3_mfd_probe(struct platform_device *pdev,
 	ds1wm_resources[0].end   >>= asic->bus_shift;
 
 	/* MMC */
-	asic->tmio_cnf = ioremap((ASIC3_SD_CONFIG_BASE >> asic->bus_shift) +
+	if (mem_sdio) {
+		asic->tmio_cnf = ioremap((ASIC3_SD_CONFIG_BASE >> asic->bus_shift) +
 				 mem_sdio->start,
 				 ASIC3_SD_CONFIG_SIZE >> asic->bus_shift);
-	if (!asic->tmio_cnf) {
-		ret = -ENOMEM;
-		dev_dbg(asic->dev, "Couldn't ioremap SD_CONFIG\n");
-		goto out;
+		if (!asic->tmio_cnf) {
+			ret = -ENOMEM;
+			dev_dbg(asic->dev, "Couldn't ioremap SD_CONFIG\n");
+			goto out;
+		}
 	}
 	asic3_mmc_resources[0].start >>= asic->bus_shift;
 	asic3_mmc_resources[0].end   >>= asic->bus_shift;