Bluetooth: Split triggering of discovery commands into separate function

The actual process of compiling the correct HCI commands for triggering
discovery is something that should be generic. So instead of mixing it
into the Start Discover operation handling, split it out into its own
function utilizing HCI request handling and just providing status in
case of errors or invalid parameters.

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