net: ethernet: mvneta: Fix error handling in mvneta_probe

When mvneta_port_power_up() fails, we should execute
cleanup functions after label err_netdev to avoid memleak.

Fixes: 41c2b6b4

 ("net: ethernet: mvneta: Add back interface mode validation")
Signed-off-by: Dinghao Liu <dinghao.liu@zju.edu.cn>
Link: https://lore.kernel.org/r/20201220082930.21623-1-dinghao.liu@zju.edu.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>