igb: Support for SFP modules discovery

This patch adds support for SFP modules media type discovery for
SGMII, which will enable driver to detect supported external PHYs,
including 100baseFXSFP module.

Signed-off-by: Akeem G Abodunrin <akeem.g.abodunrin@intel.com>
Signed-off-by: Carolyn Wyborny <carolyn.wyborny@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>