igc: Expose LPI counters

Completion to commit 900d1e8b

 ("igc: Add LPI counters")
LPI counters exposed by statistics update method.
A EEE TX LPI counter reflect the transmitter entries EEE (IEEE 802.3az)
into the LPI state. A EEE RX LPI counter reflect the receiver link
partner entries 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: Tony Nguyen <anthony.l.nguyen@intel.com>