e1000e: check down flag in tasks

This change is part of a fix to avoid any tasks running while the driver is
exiting and deinitializing resources.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>