hv_netvsc: enable multicast if necessary

My recent change to netvsc drive in how receive flags are handled
broke multicast.  The Hyper-v/Azure virtual interface there is not a
multicast filter list, filtering is only all or none. The driver must
enable all multicast if any multicast address is present.

Fixes: 009f766c

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