ACPICA: Fix compiler warnings for casting issues (only some compilers)

Fixes compiler warnings from GCC 4.2 and perhaps other compilers.
Jung-uk Kim <jkim@FreeBSD.org>

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>
Acked-by: Len Brown <len.brown@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>