Bluetooth: Convert Set Discoverable to use an asynchronous request

This patch converts Set Discoverable to use an asynchronous request
along with its own completion callback. This is necessary for splitting
raw HCI socket use cases from mgmt, as well as for enabling the hooking
up of Advertising parameters together with the HCI_DISCOVERABLE flag
(coming in later patches).

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