netfilter: nft_flow_offload: skip dst neigh lookup for ppp devices

The dst entry does not contain a valid hardware address, so skip the lookup
in order to avoid running into errors here.
The proper hardware address is filled in from nft_dev_path_info

Fixes: 72efd585 ("netfilter: flowtable: add pppoe support")
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>