Bluetooth: Add discovery type validity helper

As preparation for moving the discovery HCI commands behind
req_workqueue, add a helper and do the validity checks of the given
discovery type before proceeding further. This way we don't need to do
them again in hci_request.c.

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