asus-laptop: correct error handling in asus_read_brightness()

It is possible that acpi_evaluate_integer might fail and value would not be
set to any value so correct this defect by returning 0 in case of an
error. This is also the correct thing to return because the backlight
subsystem will print the old value of brightness in this case.

Signed-off-by: Giedrius Statkevičius <giedrius.statkevicius@gmail.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>