ixgbe: allow reading of SFF-8472 data over i2c

This patch adds functions needed for reading SFF-8472 diagnostic data
from SFP modules.

Based on original patch from Aurélien Guillaume <footplus@gmail.com>

CC: Aurélien Guillaume <footplus@gmail.com>
Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>