hv_netvsc: avoid retry on send during shutdown

Change the initialization order so that the device is ready to transmit
(ie connect vsp is completed) before setting the internal reference
to the device with RCU.

This avoids any races on initialization and prevents retry issues
on shutdown.

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