igb: Support for 100base-fx SFP

This patch adds support for 100base-fx SFP and report proper link speed/duplex
via Ethtool.

v2: fix smatch warnings

CC: Dan Carpenter <dan.carpenter@oracle.com>
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>