Bluetooth: controller: Fix Controller to Host flow control leak

When a connection is disconnected with outstanding unacked packets, the
Host has no way to signal or acknowledge their processing to the
Controller, since it is illegal to send a Host Number of Completed
Packets command when the connection is not up. Instead, consider the
outstanding packets as acked in order not to affect the correct flow
control.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>