lan78xx: fix statistics counter error

Fix rx_bytes, tx_bytes and tx_frames error in netdev.stats.
- rx_bytes counted bytes excluding size of struct ethhdr.
- tx_packets didn't count multiple packets in a single urb
- tx_bytes included 8 bytes of extra commands.

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