r8169: use new macro netif_subqueue_completed_wake in the tx cleanup path

Use new net core macro netif_subqueue_completed_wake to simplify
the code of the tx cleanup path.

Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>