netvsc: allow driver to be removed even if VF is present

If VF is attached then can still allow netvsc driver module to
be removed. Just have to make sure and do the cleanup.

Also, avoid extra rtnl round trip when calling unregister.

Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>