igb: Update NVM functions to work with i350 devices

This patch adds functions and functions pointers to accommodate
differences between NVM interfaces and options for i350 devices,
82580 devices and the rest.

Signed-off-by: Carolyn Wyborny <carolyn.wyborny@intel.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>