igb: unhide invariant returns

Return a 0 directly rather than a constant.

Reported-by: Peter Senna Tschudin <peter.senna@gmail.com>
Signed-off-by: Todd Fujinaka <todd.fujinaka@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>