hv/netvsc: fix handling of fallback to single queue mode

The netvsc device may need to fallback to running in single queue
mode if host side only wants to support single queue.

Recent change for handling mtu broke this in setup logic.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 3ffe64f1

 ("hv_netvsc: split sub-channel setup into async and sync")
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>