mlxsw: spectrum_router: Support FID-less RIFs

Loopback RIFs, which will be introduced in a follow-up patch, differ
from other RIFs in that they do not have a FID associated with them.

To support this, demote FID allocation from mlxsw_sp_rif_create to
configure op of the existing RIF types, and likewise the FID release
from mlxsw_sp_rif_destroy to deconfigure op.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>