net: ethernet: mediatek: add mtk_hw_deinit call as the opposite to mtk_hw_init call

grouping things related to the deinitialization of what
mtk_hw_init call does that help to be reused by the reset
process and the error path handling.

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