alienware-wmi: Mark DMI table as __initconst

The DMI table is only ever used during initialization. Mark it as
__initconst so its memory can be released appropriately. In turn, the
callback function can be marked with __init, too.

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>