Bluetooth: Report num supported adv instances for hw offloading

Here we make sure we properly report the number of supported
advertising slots when we are using hardware offloading. If no
hardware offloading is available, we default this value to
HCI_MAX_ADV_INSTANCES for use in software rotation as before.

This change has been tested on kukui (no ext adv) and hatch (ext adv)
chromebooks by verifying "SupportedInstances" shows 5 (the default) and
6 (slots supported by controller), respectively.

Signed-off-by: Daniel Winkler <danielwinkler@google.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>