igb: Support to read and export SFF-8472/8079 data

This patch adds support to read and export SFF-8472/8079 (SFP data)
over i2c, through Ethtool.

v2: Changed implementation to accommodate any offset within SFF module
    length boundary.

Reported-by: Aurélien Guillaume <footplus@gmail.com>
CC: Aurélien Guillaume <footplus@gmail.com>
CC: Ben Hutchings <bhutchings@solarflare.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>