mlxsw: spectrum_router: Determine offload status using generic function

The previous patch introduced a generic function to determine whether a
route should be offloaded or not. Make use of it here.

In the future we're going to add more conditions to this test (e.g.,
whether TOS is non-zero), so it makes sense to centralize it instead of
open coding it in a few places.

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