Bluetooth: Use list _safe deleting from conn chan_list

Fixes possible bug when deleting element from the list in
function hci_chan_list_flush. list_for_each_entry_rcu is used
and after deleting element from the list we also free pointer
and then list_entry_rcu is taken from freed pointer.

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>