asus-laptop: callbacks should use "driver data" parameter or field

(Changelog stolen from Alan's patch for eeepc-laptop, but this patch
does the same thing for asus-laptop)

Callback methods should not refer to a variable like "asus" (formally
"hotk").  Instead, they should extract the data they need either from
a "driver data" parameter, or the "driver data" field of the object
which they operate on.  The "asus" variable can then be removed.

In practice, drivers under "drivers/platform" can get away without using
driver data, because it doesn't make sense to have more than one
instance of them.  However this makes it harder to review them for
correctness.  This is especially true for core ACPI developers who have
not previously been exposed to this anti-pattern :-).

This will serve as an example of best practice for new driver writers
(whether they find it themselves, or have it pointed out during review
:-).
Signed-off-by: Corentin Chary <corentincj@iksaif.net>