dell-laptop: Mark dell_quirks[] DMI table as __initconst

The dell_quirks[] DMI table is only ever used during initialization.
Mark it as __initconst so its memory can be released afterwards --
roughly 5.7 kB. In turn, the callback function can be marked with
__init, too.

Also the touchpad_led_init() function can be marked __init as it's only
referenced from dell_init() -- an __init function.

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