thermal: of-thermal: Fix setting of set_emul_temp hook

Currently thermal zone set_emul_temp is set unconditionally
with of_thermal_set_emul_temp function. Set this only if the
set_emul_temp hook is provided for thermal_zone_of_device_ops.

This fixes emul_temp failures on platforms for which set_emul_temp
hook is not populated.

Fixes: "184a4bf6 (thermal: of: Extend current
	of-thermal.c code to allow setting emulated temp)"
Suggested-by: Eduardo Valentin <edubezval@gmail.com>
Signed-off-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>

Conflicts:
	drivers/thermal/of-thermal.c
Signed-off-by: Zhang Rui <rui.zhang@intel.com>