lan78xx: handle statistics counter rollover

Update to handle statistics counter rollover.
Check statistics counter periodically and compensate it when
counter value rolls over at max (20 or 32bits).

Simple mechanism adjusts monitoring timer to allow USB auto suspend.
Signed-off-by: Woojung Huh <woojung.huh@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>