mwifiex: use return value of mwifiex_add_virtual_intf() correctly

mwifiex_add_virtual_intf() returns ERR_PTR values. So use IS_ERR()
macro instead of checking for NULL pointer.
Reported-by: Ujjal Roy <royujjal@gmail.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/mwifiex/main.c

@@ -418,6 +418,7 @@ static void mwifiex_fw_dpc(const struct firmware *firmware, void *context)
 	struct mwifiex_fw_image fw;
 	struct semaphore *sem = adapter->card_sem;
 	bool init_failed = false;
+	struct wireless_dev *wdev;
 
 	if (!firmware) {
 		dev_err(adapter->dev,
@@ -474,8 +475,9 @@ static void mwifiex_fw_dpc(const struct firmware *firmware, void *context)
 
 	rtnl_lock();
 	/* Create station interface by default */
-	if (!mwifiex_add_virtual_intf(adapter->wiphy, "mlan%d",
-				      NL80211_IFTYPE_STATION, NULL, NULL)) {
+	wdev = mwifiex_add_virtual_intf(adapter->wiphy, "mlan%d",
+					NL80211_IFTYPE_STATION, NULL, NULL);
+	if (IS_ERR(wdev)) {
 		dev_err(adapter->dev, "cannot create default STA interface\n");
 		goto err_add_intf;
 	}