fw4: do not add physical devices for soft offload

fw4: do not add physical devices for soft offload

Let kernel heuristics take care of offloading decapsulation.

When software flow offloading is requested, avoid manually resolving and
adding lower physical devices to the flow table in order to let kernel
heuristics deal with the proper offloading en/decapsulation.

Fixes: https://github.com/openwrt/openwrt/issues/13410
Ref: https://github.com/openwrt/openwrt/issues/10224
Submitted-by: Andris PE <neandris@gmail.com>
[refactor code, reword commit message]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>