ACPI / debugger: Fix a redundant mutex unlock issue in acpi_aml_open()

Fix a double mutex_unlock() issue where acpi_initialize_debugger() is
called with the mutex already unlocked.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>