hv_netvsc: improve VF device matching

The code to associate netvsc and VF devices can be made less error prone
by using a better matching algorithms.

On registration, use the permanent address which avoids any possible
issues caused by device MAC address being changed. For all other callbacks,
search by the netdevice pointer value to ensure getting the correct
network device.

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