igc: add support to eeprom, registers and link self-tests

Introduced igc_diag.c and igc_diag.h, these files have the
diagnostics functionality of igc driver. For the time being
these files are being used by ethtool self-test callbacks.
Which mean that eeprom, registers and link self-tests for
ethtool were implemented.

Signed-off-by: Vitaly Lifshits <vitaly.lifshits@intel.com>
Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>