Bluetooth: Fix reference counting for LE-scan based connections

Bluetooth: Fix reference counting for LE-scan based connections

The code should never directly call hci_conn_hash_del since many
cleanup & reference counting updates would be lost. Normally
hci_conn_del is the right thing to do, but in the case of a connection
doing LE scanning this could cause a deadlock due to doing a
cancel_delayed_work_sync() on the same work callback that we were
called from.

Connections in the LE scanning state actually need very little cleanup
- just a small subset of hci_conn_del. To solve the issue, refactor
out these essential pieces into a new hci_conn_cleanup() function and
call that from the two necessary places.

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