r8169: jumbo fixes.

- fix features : jumbo frames and checksumming can not be used at the
  same time.

- introduce hw_jumbo_{enable / disable} helpers. Their content has been
  creatively extracted from Realtek's own drivers. As an illustration,
  it would be nice to know how/if the MaxTxPacketSize register operates
  when the device can work with a 9k jumbo frame as its documentation
  (8168c) can not be applied beyond ~7k.

- rtl_tx_performance_tweak is moved forward. No change.

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>