Bluetooth: Host: Refactor tx_notify, use `CONTAINER_OF`

The cast from `sys_snode_t` to `struct bt_conn_tx` assumes `node` is the
first field in `struct bt_conn_tx`. It's better to future-proof this by
using `CONTAINER_OF` to locate `node`.

Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>