e1000e: cleanup: always return 0

These are a few instances of returning a value that can only be 0 so just
use a 'return 0' to make it more obvious.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>