net: ethernet: mtk_eth_soc: support per-flow accounting on MT7988

NETSYS_V3 uses 64 bits for each counters while older SoCs are using
48/40 bits for each counter.
Support reading per-flow byte and package counters on NETSYS_V3.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/37a0928fa8c1253b197884c68ce1f54239421ac5.1690946442.git.daniel@makrotopia.org


Signed-off-by: Paolo Abeni <pabeni@redhat.com>