net: usb: support of_get_mac_address new ERR_PTR error

There was NVMEM support added to of_get_mac_address, so it could now return
ERR_PTR encoded error values, so we need to adjust all current users of
of_get_mac_address to this new fact.
Signed-off-by: Petr Štetiar <ynezz@true.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/smsc75xx.c

@@ -774,7 +774,7 @@ static void smsc75xx_init_mac_address(struct usbnet *dev)
 
 	/* maybe the boot loader passed the MAC address in devicetree */
 	mac_addr = of_get_mac_address(dev->udev->dev.of_node);
-	if (mac_addr) {
+	if (!IS_ERR(mac_addr)) {
 		memcpy(dev->net->dev_addr, mac_addr, ETH_ALEN);
 		return;
 	}

drivers/net/usb/smsc95xx.c

@@ -917,7 +917,7 @@ static void smsc95xx_init_mac_address(struct usbnet *dev)
 
 	/* maybe the boot loader passed the MAC address in devicetree */
 	mac_addr = of_get_mac_address(dev->udev->dev.of_node);
-	if (mac_addr) {
+	if (!IS_ERR(mac_addr)) {
 		memcpy(dev->net->dev_addr, mac_addr, ETH_ALEN);
 		return;
 	}