igb: use the correct i210 register for EEMNGCTL

The i210 has two EEPROM access registers that are located in
non-standard offsets: EEARBC and EEMNGCTL. EEARBC was fixed previously
and EEMNGCTL should also be corrected.

Reported-by: Roman Hodek <roman.aud@siemens.com>
Signed-off-by: Todd Fujinaka <todd.fujinaka@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>