Bluetooth: hci_conn: Make unacked packet handling more robust

This attempts to make unacked packet handling more robust by detecting
if there are no connections left then restore all buffers of the
respective pool.

Fixes: 5638d9ea ("Bluetooth: hci_conn: Fix not restoring ISO buffer count on disconnect")
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>