net: ethernet: mtk_eth_soc: fix undefined reference to `dsa_port_from_netdev'

Caused by:

 CONFIG_NET_DSA=m
 CONFIG_NET_MEDIATEK_SOC=y

mtk_ppe_offload.c:undefined reference to `dsa_port_from_netdev'

Fixes: 502e84e2

 ("net: ethernet: mtk_eth_soc: add flow offloading support")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>