Bluetooth: Add instance range check for Add Advertising command

The instance range check for Add Advertising command is missing. If the
provided instance is out of range an Invalid Parameters error should be
returned. At the moment, the generic Failed error is returned. This
extra check ensures that clear error messages are returned.

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