Bluetooth: Ensure that background scanning gets enabled on power on

The background scanning normally gets enabled during power on by
adding devices to the pending connection list. However devices
might be already on that list and the list of devices is empty,
then it is better to trigger the background manually.

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