stmmac: re-add IFF_UNICAST_FLT for dwmac1000

In commit (bfab27a1

 stmmac: add the experimental PCI support) the
IFF_UNICAST_FLT flag has been removed from the stmmac_mac_device_setup()
function. This patch re-adds the flag.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Acked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>