acpi-wmi: unsigned cannot be less than 0

include/linux/pci-acpi.h:74:

typedef u32                 acpi_status;

result is unsigned, so an error returned by acpi_bus_register_driver()
will not be noticed.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Len Brown <len.brown@intel.com>