gianfar: Handle error code at MAC address change

Handle return error code of eth_mac_addr();

Fixes: 3d23a05c

 ("gianfar: Enable changing mac addr when if up")
Signed-off-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>