net: ethernet: mtk_eth_soc: fix pse_port configuration for MT7988

MT7988 SoC support 3 NICs. Fix pse_port configuration in
mtk_flow_set_output_device routine if the traffic is offloaded to eth2.
Rely on mtk_pse_port definitions.

Fixes: 88efedf5 ("net: ethernet: mtk_eth_soc: enable nft hw flowtable_offload for MT7988 SoC")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>