Bluetooth: Check HCI_PERIODIC_INQ in start_discovery

This patch adds a HCI_PERIODIC_INQ check to start_discovery.
If periodic inquiry is enabled, we fail MGMT Start Discovery
command with MGMT_STATUS_BUSY code.

Signed-off-by: Andre Guedes <aguedespe@gmail.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo@padovan.org>