thinkpad-acpi: don't depend on led_path for led firmware type (v2)

Don't depend on the contents of led_path to know which LED interface
the firmware wants.

This removes the only user of *_path for the thinkpad-acpi ACPI
handlers, which will simplify future code.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>