net: macb: do not copy the mac address if NULL

This patch fixes the MAC address setup in the probe. The MAC address
retrieved using of_get_mac_address was checked for not containing an
error, but it may also be NULL which wasn't tested. Fix it by replacing
IS_ERR with IS_ERR_OR_NULL.

Fixes: 541ddc66 ("net: macb: support of_get_mac_address new ERR_PTR error")
Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>