thermal: rcar_thermal: Remove lock in rcar_thermal_get_current_temp()

With the ctemp value returned instead of cached in the private data
structure their is no need to take the lock when translating ctemp into
a temperature.

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/20200310170029.1648996-4-niklas.soderlund+renesas@ragnatech.se