net: ethernet: stmmac: dwmac-sun8i: enable support of unicast filtering

When adding more MAC addresses to a dwmac-sun8i interface, the device goes
directly in promiscuous mode.
This is due to IFF_UNICAST_FLT missing flag.

So since the hardware support unicast filtering, let's add IFF_UNICAST_FLT.

Fixes: 9f93ac8d ("net-next: stmmac: Add dwmac-sun8i")
Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>