thinkpad-acpi: avoid keymap pitfall

Change the code so that it will use the correct size for keymap entries.
Do it in a way that makes it harder to screw it up in the future.

Reported-by: Jaime Velasco Juan <jsagarribay@gmail.com>
Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Matthew Garrett <mjg@redhat.com>