thermal/drivers/mediatek/lvts_thermal: Guard against efuse data buffer overflow

We don't want to silently fetch garbage past the actual buffer.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20240402032729.2736685-6-nico@fluxnic.net