Bluetooth: Add data structure for advertising instance

This patch introduces a new data structure to represent advertising
instances that were added using the "Add Advertising" mgmt command.
Initially an hci_dev structure will support only one of these instances
at a time, so the current instance is simply stored as a direct member
of hci_dev.

Signed-off-by: Arman Uguray <armansito@chromium.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>