igb: Fix sparse warnings on function pointers

This patch fixes sparse warnings on function pointers that are not
defined as static.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Akeem G Abodunrin <akeem.g.abodunrin@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>