gianfar: Handle error code at MAC address change

[ Upstream commit bff5b62585123823842833ab20b1c0a7fa437f8c ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>