netvsc: fix race during initialization

When device is being setup on boot, there is a small race where
network device callback is registered, but the netvsc_device pointer
is not set yet.  This can cause a NULL ptr dereference if packet
arrives during this window.

Fixes: 46b4f7f5

 ("netvsc: eliminate per-device outstanding send counter")
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>