stmmac: dwmac-mediatek: remove the dwmac_fix_mac_speed

In current driver, MAC will always enable 2ns delay in RGMII mode,
but that's not the correct usage.

Remove the dwmac_fix_mac_speed() in driver, and recommend "rgmii-id"
for phy-mode in device tree.

Fixes: f2d356a6 ("stmmac: dwmac-mediatek: add support for mt8195")
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Biao Huang <biao.huang@mediatek.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>