igb: do a reset on SR-IOV re-init if device is down

To properly re-initialize SR-IOV it is necessary to reset the device
even if it is already down. Not doing this may result in Tx unit hangs.

Cc: stable <stable@vger.kernel.org>
Signed-off-by: Stefan Assmann <sassmann@kpanic.de>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>