r8169: use new macro netif_subqueue_maybe_stop in rtl8169_start_xmit

Use new net core macro netif_subqueue_maybe_stop in the start_xmit path
to simplify the code. Whilst at it, set the tx queue start threshold to
twice the stop threshold. Before values were the same, resulting in
stopping/starting the queue more often than needed.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>