lan78xx: add ndo_get_stats64

Add lan78xx_get_stats64 of ndo_get_stats64 to report
all statistics counters including errors from HW statistics.

Read from HW when auto suspend is disabled, use saved counter when
auto suspend is enabled because periodic call to ndo_get_stats64
prevents USB auto suspend.

Signed-off-by: Woojung Huh <woojung.huh@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>