[BLUETOOTH]: Move children of connection device to NULL before connection down.

The rfcomm tty device will possibly retain even when conn is down, and
sysfs doesn't support zombie device moving, so this patch move the tty
device before conn device is destroyed.

For the bug refered please see :
http://lkml.org/lkml/2007/12/28/87

Signed-off-by: Dave Young <hidave.darkstar@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>