ACPICA: _OSI support: Fix possible memory leak

Fixes a possible memory leak in the error exit path introduced by
recent commit 388a990 ("ACPICA: _OSI Support: handle any errors from
acpi_os_acquire_mutex()").

[rjw: Changelog]
Signed-off-by: Jung-uk Kim <jkim@FreeBSD.org>
Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>