platform/x86: thinkpad_acpi: Fix max_brightness of thinklight

Thinklight has only two values, on/off so it's reasonable for
max_brightness to be 0 and 1 as if you write anything between 0 and 255
it will be 255 anyway so there's no point for it to be 255.

This may look like it is a userspace API change, but writes with
a value larget then the new max_brightness will still be accepted,
these will be silently clamped to the new max_brightness by
led_set_brightness_nosleep(). So no userspace API problems are
expected.

Reported-by: Michał Szczepaniak <m.szczepaniak.000@gmail.com>
Link: https://lore.kernel.org/platform-driver-x86/55400326-e64f-5444-94e5-22b8214d00b6@gmail.com/
Signed-off-by: Hans de Goede <hdegoede@redhat.com>