IRQ races during change of mtu
- NAPI poll must be enabled prior to IRQ activation or the IRQ handler
  will not know what to do with an incoming packet;
- rtl8169_down() needs to try twice to sync with the IRQ handler when
  it is not issued under !netif_running() protection.

Both changes make it safe to request a change of mtu on a live device.
Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>