Bluetooth: btqca: Introduce HCI_EV_VENDOR and use it

Using HCI_VENDOR_PKT for vendor specific events does work since it has
also the value 0xff, but it is actually the packet type indicator
constant and not the event constant. So introduce HCI_EV_VENDOR and
use it.

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