thermal: db8500: Fix checking return value of thermal_zone_device_register

thermal_zone_device_register() returns ERR_PTR on error, thus use
IS_ERR rather than IS_ERR_OR_NULL to check return value.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>