net: ethernet: mtk-ppe: fix traffic offload with bridged wlan

A typical flow offload scenario for OpenWrt users is routed traffic
received by the wan interface that is redirected to a wlan device
belonging to the lan bridge. Current implementation fails to
fill wdma offload info in mtk_flow_get_wdma_info() since odev device is
the local bridge. Fix the issue running dev_fill_forward_path routine in
mtk_flow_get_wdma_info in order to identify the wlan device.

Tested-by: Paolo Valerio <pvalerio@redhat.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>