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>

drivers/net/ethernet/stmicro/stmmac/dwmac-mediatek.c

@@ -346,8 +346,6 @@ static int mediatek_dwmac_probe(struct platform_device *pdev)
 		return PTR_ERR(plat_dat);
 
 	plat_dat->interface = priv_plat->phy_mode;
-	/* clk_csr_i = 250-300MHz & MDC = clk_csr_i/124 */
-	plat_dat->clk_csr = 5;
 	plat_dat->has_gmac4 = 1;
 	plat_dat->has_gmac = 0;
 	plat_dat->pmt = 0;