Bluetooth: Update Set Discoverable to support LE

This patch updates the Set Discoverable management command to also be
applicable for LE. In particular this affects the advertising flags
where we can say "general discoverable" or "limited discoverable".

Since the device flags may not be up-to-date when the advertising data
is written this patch introduces a get_adv_discov_flags() helper
function which also looks at any pending mgmt commands (a pending
set_discoverable would be the exception when the flags are not yet
correct).

The patch also adds HCI_DISCOVERABLE flag clearing to the
mgmt_discoverable_timeout function, since the code was previously
relying on the mgmt_discoverable callback to handle this, which is only
called for the BR/EDR-only HCI_Write_Scan_Enable command.

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