hv_netvsc: cancel subchannel setup before halting device

Block setup of multiple channels earlier in the teardown
process. This avoids possible races between halt and subchannel
initialization.

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