net: ethernet: mtk_eth_soc: check max allowed hash in mtk_ppe_check_skb

Even if max hash configured in hw in mtk_ppe_hash_entry is
MTK_PPE_ENTRIES - 1, check theoretical OOB accesses in
mtk_ppe_check_skb routine

Fixes: c4f033d9 ("net: ethernet: mtk_eth_soc: rework hardware flow table management")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>