igc: Add Energy Efficient Ethernet ability

According to the IEEE standard report the EEE ability (registers 7.60 and
7.62) and the EEE Link Partner ability (registers 7.61 and 7.63). Use the
kernel's 'ethtool_keee' structure and report EEE link modes.

Example:
ethtool --show-eee <device>

Before:
Advertised EEE link modes:  Not reported
Link partner advertised EEE link modes:  Not reported

After:
	Advertised EEE link modes:  100baseT/Full
	                            1000baseT/Full
	                            2500baseT/Full
	Link partner advertised EEE link modes:  100baseT/Full
	                                         1000baseT/Full
	                                         2500baseT/Full

Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Tested-by: Avigail Dahan <avigailx.dahan@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>