igc: Add LPI counters

Add EEE TX LPI and EEE RX LPI counters. A EEE TX LPI event
occurs when the transmitter enters EEE (IEEE 802.3az) LPI
state. A EEE RX LPI event occurs when the receiver detect
link partner entry into EEE(IEEE 802.3az) LPI state.

Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>