thermal: rcar_thermal: Handle probe error gracefully

If the common register memory resource is not available the driver needs
to fail gracefully to disable PM. Instead of returning the error
directly store it in ret and use the already existing error path.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200310114709.1483860-1-niklas.soderlund+renesas@ragnatech.se