ACPICA: Dispatcher: Fix an unbalanced lock exit path in acpi_ds_auto_serialize_method()

There is a lock unbalanced exit path in acpi_ds_initialize_method(),
this patch corrects it.

Fixes: 441ad11d

 (ACPICA: Dispatcher: Fix a mutex issue for method auto serialization)
Tested-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>