xfrm: remove eth_proto value from xfrm_state_afinfo

xfrm_prepare_input needs to lookup the state afinfo backend again to fetch
the address family ethernet protocol value.

There are only two address families, so a switch statement is simpler.
While at it, use u8 for family and proto and remove the owner member --
its not used anywhere.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>