Bluetooth: Move connectable changes to hdev->req_workqueue

This way the connectable changes are synchronized against each other,
which helps avoid potential races. The connectable mode is also linked
together with LE advertising which makes is more convenient to have it
behind the same workqueue.

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