thermal/drivers/devfreq_cooling: Fix wrong return on error path

The following error is reported by kbuild:

 smatch warnings:
 drivers/thermal/devfreq_cooling.c:433 of_devfreq_cooling_register_power() warn: passing zero to 'ERR_PTR'

Fix the error code by the setting the 'err' variable instead of 'cdev'.

Fixes: f8d354e8 ("thermal/drivers/devfreq_cooling: Use device name instead of auto-numbering")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20210319202424.890968-1-daniel.lezcano@linaro.org