hv_netvsc: dev hold/put reference to VF

The netvsc driver holds a pointer to the virtual function network device if
managing SR-IOV association. In order to ensure that the VF network device
does not disappear, it should be using dev_hold/dev_put to get a reference
count.

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