net: ethernet: mtk_eth_soc: fix L2 offloading with DSA untag offload

Check for skb metadata in order to detect the case where the DSA header
is not present.

Fixes: 2d7605a7

 ("net: ethernet: mtk_eth_soc: enable hardware DSA untagging")
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Link: https://lore.kernel.org/r/20230330120840.52079-2-nbd@nbd.name
Signed-off-by: Jakub Kicinski <kuba@kernel.org>