thermal: rcar_thermal: Do not store ctemp in rcar_thermal_priv

There is no need to cache the ctemp value in the private data structure
as it's always prefetched before it's used. Remove it from the structure
and have rcar_thermal_update_temp return the value instead of storing
it.

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/20200310170029.1648996-3-niklas.soderlund+renesas@ragnatech.se