ACPI: Fix crash on ASUS laptops

This patch fixes the crash I experienced in 2.6.29-rc2.
Tested on ASUS M50vm.

Signed-off-by: Tero Roponen <tero.roponen@gmail.com>
Acked-by: Alexey Starikovskiy <astarikovskiy@suse.de>
Signed-off-by: Len Brown <len.brown@intel.com>