thinkpad-acpi: detect EC node using its HID (v2)

Use the EC HID (PNP0C09) to locate its main node, instead of a static
list.

Suggested-by: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Cc: Matthew Garrett <mjg@redhat.com>