Bluetooth: Add HCI command structure for writing current IAC LAP

This patch just adds the HCI command structure for configuring the
current IAC LAP setting. The length of the command is variable and
supports more than two IAC. However since there is only general
discoverable and limited discoverable modes, this can be limited
to two possible IACs.

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