staging: wfx: fix Oops when CONFIG_OF_NET is not set

In most case, of_get_mac_address() return NULL in case of error.
However, if CONFIG_OF_NET is not set, it return -ENODEV.
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20191017093954.657-4-Jerome.Pouiller@silabs.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/wfx/main.c

@@ -407,7 +407,7 @@ int wfx_probe(struct wfx_dev *wdev)
 	for (i = 0; i < ARRAY_SIZE(wdev->addresses); i++) {
 		eth_zero_addr(wdev->addresses[i].addr);
 		macaddr = of_get_mac_address(wdev->dev->of_node);
-		if (macaddr) {
+		if (!IS_ERR_OR_NULL(macaddr)) {
 			ether_addr_copy(wdev->addresses[i].addr, macaddr);
 			wdev->addresses[i].addr[ETH_ALEN - 1] += i;
 		}