igc: Expose LPI counters

Expose EEE Tx and Rx low power idle counters via ethtool
A EEE TX or RX LPI event occurs when the transmitter or the receiver
enters EEE (IEEE802.3az) LPI state.
ethtool --statistics <iface>

Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Tested-by: Dvora Fuxbrumer <dvorax.fuxbrumer@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>