Bluetooth: Fix clearing flags on power off before notifying mgmt

When powering off the device the hdev->flags and hdev->dev_flags need to
be cleared before calling mgmt_powered(). If this is not done the
resulting events sent to user space may contain incorrect values.

Note that the HCI_AUTO_OFF flag accessed right after this is part of the
persistent flags, so it's unchanged by the hdev->dev_flags reset.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>