asus-laptop: Fix is_visible return value

With a Lucid platform, asus_sysfs_is_visible() returned a boolean for
ls_switch and ls_level attributes.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>