net: stmmac: fix missing IFF_MULTICAST check in dwmac4_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: 477286b5 ("stmmac: add GMAC4 core support")
Signed-off-by: Verma, Aashish <aashishx.verma@intel.com>
Tested-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>