Bluetooth: Wait for HCI command completion with mgmt_set_powered

We should only notify user space that the adapter has been powered on
after all HCI commands related to the action have completed. This patch
fixes the issue by instating an async request complete callback for
these HCI commands and only notifies user space in the callback.

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>