Bluetooth: Host: Add missing `bt_tx_irq_raise()`

Trigger the TX processor on connection teardown.

When a disconnection happens before the controller has acknowledged some
ACL fragments the host has sent, we run `process_unack_tx()` to free
those unacknowledged buffers and their associated TX contexts.

The problem is that the TX processor still holds a reference to the conn
object. That reference is not released until the TX processor is
triggered again and figures out that the connection is invalid.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>