powerpc: tsi108: 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/powerpc/sysdev/tsi108_dev.c

@@ -105,7 +105,7 @@ static int __init tsi108_eth_of_init(void)
 		}
 
 		mac_addr = of_get_mac_address(np);
-		if (mac_addr)
+		if (!IS_ERR(mac_addr))
 			memcpy(tsi_eth_data.mac_addr, mac_addr, 6);
 
 		ph = of_get_property(np, "mdio-handle", NULL);