net: ethernet: mtk_ppe: fix possible NULL pointer dereference in mtk_flow_get_wdma_info

odev pointer can be NULL in mtk_flow_offload_replace routine according
to the flower action rules. Fix possible NULL pointer dereference in
mtk_flow_get_wdma_info.

Fixes: a333215e ("net: ethernet: mtk_eth_soc: implement flow offloading to WED devices")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/4e1685bc4976e21e364055f6bee86261f8f9ee93.1658137753.git.lorenzo@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>