net: dsa: felix: use ocelot's ndo_get_stats64 method

Move the logic from the ocelot switchdev driver's ocelot_get_stats64()
method to the common switch lib and reuse it for the DSA driver.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>