net: dsa: microchip: ksz9477: export HW stats over stats64 interface

Provide access to HW offloaded packets over stats64 interface.
The rx/tx_bytes values needed some fixing since HW is accounting size of
the Ethernet frame together with FCS.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>