ixgbe: cleanup some minor issues in ixgbe_down()

This patch cleans up two minor issues in ixgbe_down.  Specifically it
addresses the fact that the VFs should not be pinged until after interrupts
are disabled otherwise they might still get a response.  It also drops the
use of the txdctl temporary variable since the only bit we should be
writing to the TXDCTL registers during a shutdown is the flush bit.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>