mlxsw: spectrum_router: Move mlxsw_sp_rif_ipip_lb_op()

Move the function so that it can be called without forward declaration
from a function that will be added in a follow-up patch.

Fixes: 0063587d

 ("mlxsw: spectrum: Support decap-only IP-in-IP tunnels")
Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>