sony-laptop: don't change keyboard backlight settings

Do not touch keyboard backlight unless explicitly passed a module
parameter.  In this way we won't make wrong assumptions about what are
good default values since they actually are different from model to
model.

The only side effect is that we won't know what is the current value
until set via the sysfs attributes.

Signed-off-by: Mattia Dongili <malattia@linux.it>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>