Bluetooth: Run all background scan updates through req_workqueue

Instead of firing off a simple async request queue all background scan
updates through req_workqueue and use hci_req_sync() there to ensure
that no two updates overlap with each other.

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