ARM: Kirkwood: 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>

arch/arm/mach-mvebu/kirkwood.c

@@ -92,7 +92,8 @@ static void __init kirkwood_dt_eth_fixup(void)
 			continue;
 
 		/* skip disabled nodes or nodes with valid MAC address*/
-		if (!of_device_is_available(pnp) || of_get_mac_address(np))
+		if (!of_device_is_available(pnp) ||
+		    !IS_ERR(of_get_mac_address(np)))
 			goto eth_fixup_skip;
 
 		clk = of_clk_get(pnp, 0);