hv_netvsc: Fix race of register_netdevice_notifier and VF register

If VF NIC is registered earlier, NETDEV_REGISTER event is replayed,
but NETDEV_POST_INIT is not.

Move register_netdevice_notifier() earlier, so the call back
function is set before probing.

Cc: stable@vger.kernel.org
Fixes: e04e7a7b

 ("hv_netvsc: Fix a deadlock by getting rtnl lock earlier in netvsc_probe()")
Reported-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Reviewed-by: Wojciech Drewek <wojciech.drewek@intel.com>
Reviewed-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>