nimble/l2cap: Disconnect CoC when too big SDU is sent by peer

This is to satisfy PTS and we actually planned to do so anyway.

nimble/host/src/ble_l2cap_coc.c

@@ -199,9 +199,11 @@ ble_l2cap_coc_rx_fn(struct ble_l2cap_chan *chan)
 
         sdu_len = get_le16((*om)->om_data);
         if (sdu_len > rx->mtu) {
-            /* TODO Disconnect?*/
             BLE_HS_LOG(INFO, "error: sdu_len > rx->mtu (%d>%d)\n",
                        sdu_len, rx->mtu);
+
+            /* Disconnect peer with invalid behaviour */
+            ble_l2cap_disconnect(chan);
             return BLE_HS_EBADDATA;
         }