ACPICA: Fix possible memory leak in Unload() operator

The DdbHandle returned by Load() does not have its reference count
decremented during unload, leading to a memory leak. Lin Ming.

Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Andi Kleen <ak@linux.intel.com>