ixgbe: Two small fixes for 82599 when bringing the device down and for WoL

The Tx DMA unit should be disabled when bringing the device down.  Also,
the KX4 device with 82599 supports WoL, so we should clear the Wake Up
Status (WUS) after a PCIe slot reset.

Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Acked-by: Mallikarjuna R Chilakala <mallikarjuna.chilakala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>