mlxsw: core_thermal: Set THERMAL_TRIP_FLAG_RW_TEMP directly

It is now possible to flag trip points with THERMAL_TRIP_FLAG_RW_TEMP
to allow their temperature to be set from user space via sysfs instead
of using a nonzero writable trips mask during thermal zone registration,
so make the mlxsw code do that.

No intentional functional impact.

Note that this change is requisite for dropping the mask argument from
thermal_zone_device_register_with_trips() going forward.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Daniel Lezcano <daniel.lezcano@linaro.org>