mlxsw: spectrum_router: Change mlxsw_sp_ipip_lb_ul_vr_id()

For the sake of Spectrum-2 GRE support, as ul_vr_id field is reserved for
Spectrum-2, Change mlxsw_sp_ipip_lb_ul_vr_id() implementation not to use
the reserved field.

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