ACPICA: Silence the warning about _BIF returning the buffer

_BIF was returning buffer instead of a string since day 1 of ACPI.
Adding a warning for that is noble, but people don't like
when someone cries wolf in a production system.

Reference: http://bugzilla.kernel.org/show_bug.cgi?id=14379


Signed-off-by: Alexey Starikovskiy <astarikovskiy@suse.de>
Signed-off-by: Len Brown <len.brown@intel.com>