net: ethernet: mediatek: use [get|set]_link_ksettings

1) use new api [get|set]_link_ksettings instead
of [get|set]_settings old ones.

2) dev->phydev is sure being ready before calling
these callbacks, so removing all the sanity check
if it is existing.

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