igb: Refractoring function pointers in igb_get_invariants function

This patch simplifies igb_get_invariants function by moving all implemented
function pointers in this function to individual separate functions,
based on their functionalities, this would make debugging much easier.

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>