thinkpad_acpi: return -NODEV while operating uninitialized LEDs

Not all 0-15 LEDs are available for all models, sometimes it's even not
safe. This patch return -NODEV while operating uninitialized LEDs.

Signed-off-by: Adam Lee <adam.lee@canonical.com>
Acked-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>