Bluetooth: Use req_workqueue for background scanning when powering on

We can easily use the new req_workqueue based background scan update
for the power on case. This also removes the last external user of
__hci_update_background_scan().

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