net: ethernet: mediatek: cleanup error path inside mtk_hw_init

This cleans up the error path inside mtk_hw_init call, causing it able
to exit appropriately when something fails and also includes refactoring
mtk_cleanup call to make the partial logic reusable on the error path.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>