Bluetooth: hci_sync: Fix not disabling advertising instance

As the code comments on hci_setup_ext_adv_instance_sync suggests the
advertising instance needs to be disabled in order to update its
parameters, but it was wrongly checking that !adv->pending.

Fixes: cba6b758 ("Bluetooth: hci_sync: Make use of hci_cmd_sync_queue set 2")
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>