Bluetooth: Fix returning proper command status for start_discovery

Management commands should whenever possible fail with proper command
status or command complete events. This patch fixes the
mgmt_start_discovery command to do this for the failure cases where an
incorrect parameter value was passed to it ("not supported" if the
parameter value was valid but the controller doesn't support it and
"invalid params" if it isn't valid at all).

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>