hv_netvsc: Cleanup the test for freeing skb when we use sendbuf mechanism

In preparation for embedding the rndis state and other packet state into
the skb, cleanup the test for freeing the skb.

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