igb: Fix SGMII SFP module discovery for 100FX/LX.

commit 5365ec1a

 upstream.

Changing the link mode should also be done for 100BaseFX SGMII modules,
otherwise they just don't work when the default link mode in CTRL_EXT
coming from the EEPROM is SERDES.

Additionally 100Base-LX SGMII SFP modules are also supported now, which
was not the case before.

Tested with an i210 using Flexoptix S.1303.2M.G 100FX and
S.1303.10.G 100LX SGMII SFP modules.

Signed-off-by: Manfred Rudigier <manfred.rudigier@omicronenergy.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>