thermal: netlink: Use for_each_trip() in thermal_genl_cmd_tz_get_trip()

Make thermal_genl_cmd_tz_get_trip() use for_each_trip() instead of an open-
coded loop over trip indices.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>