net: stmmac: dwxgmac: Finish the Flow Control implementation

Finish the implementation of Flow Control feature. In order for it to
work correctly we need to set EHFC bit and the correct threshold values
for activating and deactivating it.

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Cc: Joao Pinto <jpinto@synopsys.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>