Bluetooth: Update background scanning from hci_conn_params_clear

When calling hci_conn_params_clear function, it should update the
background scanning properly and not require a separate call to
update it.

For the case when the function is used during unregister of a
controller, an extra safe guard is but in place.

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