thermal: rcar: fix missing unlock on error in rcar_thermal_update_temp()

Add the missing unlock before return from function rcar_thermal_update_temp()
in the error handling case.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>