hv_netvsc: introduce {net, hv}_device_to_netvsc_device() helpers

Make it easier to get 'struct netvsc_device' from 'struct net_device' and
'struct hv_device' by introducing inline helpers.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>