thinkpad-acpi: lock down size of hotkey keymap

Use a safer coding style for the hotkey keymap.  This does not fix any
problems, as the current code is correct.  But it might help avoid
mistakes in the future.

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