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>