igc: Remove the internal 'eee_advert' field

Since the kernel's 'ethtool_keee' structure is in use, the internal
'eee_advert' field becomes pointless and can be removed.

This patch comes to clean up this redundant code.

Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Tested-by: Mor Bar-Gabay <morx.bar.gabay@intel.com> (A Contingent Worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>