thermal: power_allocator: exit early if there are no cooling devices

Don't waste cycles in the power allocator governor's throttle function
if there are no cooling devices and exit early.

This commit doesn't change any functionality, but should provide better
performance for the odd case of a thermal zone with trip points but
without cooling devices.

Cc: Zhang Rui <rui.zhang@intel.com>
Cc: Eduardo Valentin <edubezval@gmail.com>
Reviewed-by: NDaniel Kurtz <djkurtz@chromium.org>
Signed-off-by: NJavi Merino <javi.merino@arm.com>
Signed-off-by: NEduardo Valentin <edubezval@gmail.com>

drivers/thermal/power_allocator.c

@@ -346,6 +346,11 @@ static int allocate_power(struct thermal_zone_device *tz,
 		}
 	}
 
+	if (!num_actors) {
+		ret = -ENODEV;
+		goto unlock;
+	}
+
 	/*
 	 * We need to allocate five arrays of the same size:
 	 * req_power, max_power, granted_power, extra_actor_power and