Bluetooth: Read number of supported IAC on controller setup

When initializing a controller make sure to read out the number of
supported IAC and store its result. This value is needed to determine
if limited discoverable for BR/EDR can be configured or not.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>