Bluetooth: hci_conn: Always allocate unique handles

This attempts to always allocate a unique handle for connections so they
can be properly aborted by the likes of hci_abort_conn, so this uses the
invalid range as a pool of unset handles that way if userspace is trying
to create multiple connections at once each will be given a unique
handle which will be considered unset.

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>