drivers: thermal: processor_thermal: Downgrade error message

Downgrade "Unsupported event" message from dev_err to dev_dbg. Otherwise it
floods with this message one some platforms.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>