Bluetooth: hci_sync: Avoid adding default advertising on startup

Bluetooth: hci_sync: Avoid adding default advertising on startup

list_empty(&hdev->adv_instances) is always true during startup,
so an advertising instance is added by default.

Call trace:
  dump_backtrace+0x94/0xec
  show_stack+0x18/0x24
  dump_stack_lvl+0x48/0x60
  dump_stack+0x18/0x24
  hci_setup_ext_adv_instance_sync+0x17c/0x328
  hci_powered_update_adv_sync+0xb4/0x12c
  hci_powered_update_sync+0x54/0x70
  hci_power_on_sync+0xe4/0x278
  hci_set_powered_sync+0x28/0x34
  set_powered_sync+0x40/0x58
  hci_cmd_sync_work+0x94/0x100
  process_one_work+0x168/0x444
  worker_thread+0x378/0x3f4
  kthread+0x108/0x10c
  ret_from_fork+0x10/0x20

Link: https://github.com/bluez/bluez/issues/1442
Signed-off-by: Yang Li <yang.li@amlogic.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>