igb: Alternate MAC Address EEPROM Updates

This code check word 0x37 in the EEPROM, if it is 0xFFFF _or_ 0x0000, then
there is no Alternate MAC Address in the EEPROM.

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>