net: ethernet: mtk_eth_soc: Fix packet statistics support for MT7628/88

The MT7628/88 SoC(s) have other (limited) packet counter registers than
currently supported in the mtk_eth_soc driver. This patch adds support
for reading these registers, so that the packet statistics are correctly
updated.

Additionally the defines for the non-MT7628 variant packet counter
registers are added and used in this patch instead of using hard coded
values.

Signed-off-by: Stefan Roese <sr@denx.de>
Fixes: 296c9120 ("net: ethernet: mediatek: Add MT7628/88 SoC support")
Cc: Felix Fietkau <nbd@nbd.name>
Cc: John Crispin <john@phrozen.org>
Cc: Ilya Lipnitskiy <ilya.lipnitskiy@gmail.com>
Cc: Reto Schneider <code@reto-schneider.ch>
Cc: Reto Schneider <reto.schneider@husqvarnagroup.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: David S. Miller <davem@davemloft.net>