Bluetooth: hci_conn: Fix not restoring ISO buffer count on disconnect

When disconnecting an ISO link the controller may not generate
HCI_EV_NUM_COMP_PKTS for unacked packets which needs to be restored in
hci_conn_del otherwise the host would assume they are still in use and
would not be able to use all the buffers available.

Fixes: 26afbd82 ("Bluetooth: Add initial implementation of CIS connections")
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Tested-by: Frédéric Danis <frederic.danis@collabora.com>