[Bluetooth] Fix reference counting during ACL config stage

The ACL config stage keeps holding a reference count on incoming
connections when requesting the extended features. This results in
keeping an ACL link up without any users. The problem here is that
the Bluetooth specification doesn't define an ownership of the ACL
link and thus it can happen that the implementation on the initiator
side doesn't care about disconnecting unused links. In this case the
acceptor needs to take care of this.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>