fujitsu-tablet: Mark DMI callbacks as __init code

The DMI table is already marked as __initconst, so can be the callback
functions as they're only used in that context.

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Cc: Robert Gerlach <khnz@gmx.de>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>