thermal: rcar: check irq possibility in rcar_thermal_irq_xxx()

Current rcar thermal driver sometimes checks irq possibility when it
calls rcar_thermal_irq_enable/disable(), but sometimes not.
This patch checks it inside rcar_thermal_irq_enable/disable().

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>