ixgbe: refresh the ixgbe_down function

clean up the hardware shutdown sequence to prevent hardware
from continuing to send when resetting or unloading.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>