Bluetooth: mgmt: Allow connectable/discoverable changes in off state

This patch makes it possible to toggle the connectable & discoverable
settings when powered off. Two new hdev->dev_flags flags are added to
track what the scan mode should be when the device is finally powered
on.

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