thermal/drivers/rcar_gen3: Use the generic function to get the number of trips

The thermal core framework allows to get the number of thermal trips,
use it instead of visiting the thermal core structure internals.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/20221003092602.1323944-16-daniel.lezcano@linaro.org