net: stmmac: xgmac: fix missing IFF_MULTICAST checki in dwxgmac2_set_filter

Without checking for IFF_MULTICAST flag, it is wrong to assume multicast
filtering is always enabled. By checking against IFF_MULTICAST, now
the driver behaves correctly when the multicast support is toggled by below
command:-
  ip link set <devname> multicast off|on

Fixes: 0efedbf1 ("net: stmmac: xgmac: Fix XGMAC selftests")
Signed-off-by: Tan, Tee Min <tee.min.tan@intel.com>
Signed-off-by: Ong Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>