igb: Fix possible panic caused by Rx traffic arrival while interface is down

This patch reorders disabling napi and irqs during igb_down.
This is done to avoid possible panic's found in other Intel drivers
when Rx traffic arrives while interface is going down.

Signed-off-by: Carolyn Wyborny <carolyn.wyborny@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>