Bluetooth: AMP: Drop packets when no l2cap conn exist

High Speed hci_conn should always have l2cap_conn associated with it.

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>