igb: Updates to read nvm function for i211 device

The i211 has an integrated secure space to store configuration information that is
usually stored in an EEPROM or flash type device. This patch updates the
read functions to return values or appropriate error codes to prevent
unnecessary init failures on some configuration schemes.

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>