vnic: report that the driver supports multicast

The driver is currently hardcoded to force promiscuous mode, so all of
the MAC filtering code is presently unused and multicast should "just
work."  Report to the higher layers that multicast is supported.

PR:		223573
Reported by:	bz
Sponsored by:	The FreeBSD Foundation, Packet.net (hardware)

sys/dev/vnic/nicvf_main.c

@@ -350,7 +350,7 @@ nicvf_setup_ifnet(struct nicvf *nic)
 
 	if_setsoftc(ifp, nic);
 	if_initname(ifp, device_get_name(nic->dev), device_get_unit(nic->dev));
-	if_setflags(ifp, IFF_BROADCAST | IFF_SIMPLEX);
+	if_setflags(ifp, IFF_BROADCAST | IFF_SIMPLEX | IFF_MULTICAST);
 
 	if_settransmitfn(ifp, nicvf_if_transmit);
 	if_setqflushfn(ifp, nicvf_if_qflush);