igb: Add support of SerDes Forced mode for certain hardware

This patch changes the serdes link code to support a forced mode for
some hardware, based on bit set in EEPROM.

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>