net-next: stmmac: dwmac-mediatek: remove fine-tune property

1. remove fine-tune property and related setting to simplify
the timing adjustment flow.
2. set timing value according to the value from device tree,
and will not care whether PHY insert internal delay.

Signed-off-by: Biao Huang <biao.huang@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>