net: stmmac: dwmac-mediatek: modify csr_clk value to fix mdio read/write fail

1. the frequency of csr clock is 66.5MHz, so the csr_clk value should
be 0 other than 5.
2. the csr_clk can be got from device tree, so remove initialization here.

Fixes: 9992f37e ("stmmac: dwmac-mediatek: add support for mt2712")
Signed-off-by: Biao Huang <biao.huang@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>