Bluetooth: host: Fix ordering of TX sent callbacks

Now that the TX callbacks happen from the system workqueue but fixed
channels get processed from the RX thread there's a risk that the
ordering of these gets messed up. This is particularly bad for ATT
when it's trying to enforce flow control.

To fix the issue store the completed TX packet information in a
per-connection list and process this list before processing any new
packets for the same connection. We still also schedule a workqueue
callback, which will simply do nothing for this list if bt_recv()
already took care of it.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>