net: ethernet: stmmac: dwmac-sun8i: Enable control of loopback

This patch enable use of set_mac_loopback in dwmac-sun8i

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
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>