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: Sonic Zhang <sonic.zhang@analog.com>
Acked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Tested on LPC4350 Hitex board.

Fixes: f748be53

 ("stmmac: support new GMAC4")
Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Tested-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>