ACPI Exception (video-1721): UNKNOWN_STATUS_CODE, Cant attach device

The child of a video bus device is not alway a video device.
It should be a warn message rather than an exception here.
http://bugzilla.kernel.org/show_bug.cgi?id=9761

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>