thermal/debugfs: Unlock on error path in thermal_debug_tz_trip_up()

Add a missing mutex_unlock(&thermal_dbg->lock) to this error path.

Fixes: 7ef01f22 ("thermal/debugfs: Add thermal debugfs information for mitigation episodes")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>