platform/x86: thinkpad_acpi: Fix thinklight LED brightness returning 255

Reading the thinklight LED brightnes while the LED is on returns
255 (LED_FULL) but we advertise a max_brightness of 1, so this should
be 1 (LED_ON).

Fixes: db5e2a4c

 ("platform/x86: thinkpad_acpi: Fix max_brightness of thinklight")
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20230127235723.412864-1-hdegoede@redhat.com