fw4: improve flowtable handling

 - Delete the flowtable while loading the rulset in case it exists already
   since flowtable with offload flag canot overwrite ones without and vice
   versa

 - Resolve higher level devices such as 802.1q or bridge devices to lower,
   offload capable ones in case hardware offloading is requested

 - Revert disabling of "flow_offloading_hw" option

Signed-off-by: Jo-Philipp Wich <jo@mein.io>