net: ethernet: mtk_star_emac: use devm_register_netdev()

Use the new devres variant of register_netdev() in the mtk-star-emac
driver and shrink the code by a couple lines.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>