stmmac: hardware TX COE doesn't work when force_thresh_dma_mode is set

Commit f748be53 ("stmmac: support new GMAC4") reverted a previous fix
by mistake. This commit re-applies said fix:

  commit dec2165f
  Author: Sonic Zhang <sonic.zhang@analog.com>
  Date:   Thu Jan 22 14:55:57 2015 +0800
  stmmac: hardware TX COE doesn't work when force_thresh_dma_mode is set

  Clear the TX COE bit when force_thresh_dma_mode is set even hardware
  dma capability says support.

  Tested on BF609.
Signed-off-by: NSonic Zhang <sonic.zhang@analog.com>
Acked-by: NGiuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Tested on LPC4350 Hitex board.

Fixes: f748be53 ("stmmac: support new GMAC4")
Signed-off-by: NEzequiel Garcia <ezequiel@vanguardiasur.com.ar>
Tested-by: NJoachim Eastwood <manabian@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

@@ -3131,7 +3131,12 @@ static int stmmac_hw_init(struct stmmac_priv *priv)
 		priv->plat->enh_desc = priv->dma_cap.enh_desc;
 		priv->plat->pmt = priv->dma_cap.pmt_remote_wake_up;
 
-		priv->plat->tx_coe = priv->dma_cap.tx_coe;
+		/* TXCOE doesn't work in thresh DMA mode */
+		if (priv->plat->force_thresh_dma_mode)
+			priv->plat->tx_coe = 0;
+		else
+			priv->plat->tx_coe = priv->dma_cap.tx_coe;
+
 		/* In case of GMAC4 rx_coe is from HW cap register. */
 		priv->plat->rx_coe = priv->dma_cap.rx_coe;