hv_netvsc: fix filter flags

The recent change to not always enable all multicast and broadcast
was broken; meant to set filter, not change flags.

Fixes: 009f766c ("hv_netvsc: filter multicast/broadcast")
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>