thermal/drivers/mediatek/lvts: Fix parameter check in lvts_debugfs_init()

The documentation says "If an error occurs, ERR_PTR(-ERROR) will be
returned" but the current code checks against a NULL pointer returned.

Fix this by checking if IS_ERR().

Signed-off-by: Minjie Du <duminjie@vivo.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20230713042413.2519-1-duminjie@vivo.com