sunvnet: free pending tx buffers before clearing ring data

This patch moves the clearing of ring data in vnet_port_free_tx_bufs to after
 the freeing of pending buffers in the ring. Otherwise, this can result in
 dereferencing a NULL pointer.

Reported-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Signed-off-by: David L Stevens <david.stevens@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>