openvswitch: Unify vport error stats handling.

Following patch changes vport->send return type so that vport
layer can do error accounting.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>

net/openvswitch/vport.h

@@ -123,7 +123,8 @@ struct vport_parms {
  * existing vport to a &struct sk_buff.  May be %NULL for a vport that does not
  * have any configuration.
  * @get_name: Get the device's name.
- * @send: Send a packet on the device.  Returns the length of the packet sent.
+ * @send: Send a packet on the device.  Returns the length of the packet sent,
+ * zero for dropped packets or negative for error.
  */
 struct vport_ops {
 	enum ovs_vport_type type;