hv_netvsc: Fix the order of num_sc_offered decrement

Reorder the code in netvsc_sc_open(), so num_sc_offered is only decremented
after vmbus_open() is called. This avoid pontential race of removing device
before all channels are setup.

Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Reviewed-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>