virtio: Fix typo in virtio_serial_init()

Fixes: 300384f3dc68 ("virtio: Store queue descriptors in virtio_device")
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>