e1000e: alternate MAC address update

If word 0x37 in the EEPROM is 0xFFFF _or_ 0x0000, then there is no
alternate MAC address in the EEPROM.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>