Bluetooth: Add timer to force power off

If some of the cleanup commands caused by mgmt_set_powered(off) never
complete we should still force the adapter to be powered down. This is
rather easy to do since hdev->power_off is already a delayed work
struct. This patch schedules this delayed work if at least one HCI
command was sent by the cleanup procedure.

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