mlxsw: spectrum_router: Avoid allocating NH counters twice

mlxsw_sp_nexthop_counter_disable() decays to a nop when called on a
disabled counter, but mlxsw_sp_nexthop_counter_enable() can't similarly
be called on an enabled counter. This would be useful in the following
patches. Add the missing condition.
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://lore.kernel.org/r/0cc9050e196366c1387ab5ee47f1cee8ecde9c86.1709901020.git.petrm@nvidia.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/mellanox/mlxsw/spectrum_router.c

@@ -3157,6 +3157,9 @@ int mlxsw_sp_nexthop_counter_enable(struct mlxsw_sp *mlxsw_sp,
 	struct devlink *devlink;
 	int err;
 
+	if (nh->counter_valid)
+		return 0;
+
 	devlink = priv_to_devlink(mlxsw_sp->core);
 	if (!devlink_dpipe_table_counter_enabled(devlink,
 						 MLXSW_SP_DPIPE_TABLE_NAME_ADJ))