thermal/drivers/mediatek/lvts_thermal: Check NULL ptr on lvts_data

Verify that lvts_data is not NULL before using it.

Signed-off-by: Julien Panis <jpanis@baylibre.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20240502-mtk-thermal-lvts-data-v1-1-65f1b0bfad37@baylibre.com