thermal: trip: Drop redundant trips check from for_each_thermal_trip()

It is invalid to call for_each_thermal_trip() on an unregistered thermal
zone anyway, and as per thermal_zone_device_register_with_trips(), the
trips[] table must be present if num_trips is greater than zero for the
given thermal zone.

Hence, the trips check in for_each_thermal_trip() is redundant and so it
can be dropped.

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