Bluetooth: l2CAP_BR: Fix the response cannot be sent out issue

There is an issue that the L2CAP connect response function
`l2cap_br_conn_req_reply()` is called first. And then the ACL
disconnect function `bt_conn_disconnect()` is called following. But
the packet of `bt_conn_disconnect()` appears on the HCI bus first.

Add a flag `L2CAP_FLAG_DISCONNECT_ACL` to flag whether it is needed to
disconnect ACL connection.

Set the flag `L2CAP_FLAG_DISCONNECT_ACL` when result of the L2CAP
channel conn rsp is `BT_L2CAP_BR_ERR_SEC_BLOCK`.

Add a l2cap packet sent out callback `l2cap_br_conn_rsp_sent_cb()`.
When the callback triggered, disconnect the ACL connect with
`BT_HCI_ERR_AUTH_FAIL` if the flag `L2CAP_FLAG_DISCONNECT_ACL` has
been set.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>