Bluetooth: Move discoverable changes to hdev->req_workqueue

The discoverable mode is intrinsically linked with the connectable
mode e.g. through sharing the same HCI command (Write Scan Enable) for
BR/EDR. It makes therefore sense to move it to hci_request.c and run
the changes through the same hdev->req_workqueue.

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