thermal/core: fix error code in __thermal_cooling_device_register()

Return an error pointer if ->get_max_state() fails.  The current code
returns NULL which will cause an oops in the callers.

Fixes: c408b3d1

 ("thermal: Validate new state in cur_state_store()")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>