Bluetooth: Add new state HCI_POWERING_DOWN

Add a new state HCI_POWERING_DOWN that indicates that the device is
currently powering down, this will be useful for the next commit.

Signed-off-by: Jonas Dreßler <verdre@v0yd.nl>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>