virtio_pci(4): Fix typo in read_ivar method

Prior to this revision, vtpci's BUS_READ_IVAR method on VIRTIO_IVAR_SUBVENDOR
accidentally returned the PCI subdevice.

The typo seems to have been introduced with the original commit adding
VIRTIO_IVAR_{{SUB,}DEVICE,{SUB,}VENDOR} to virtio_pci.  The commit log and code
strongly suggest that the ivar was intended to return the subvendor rather than
the subdevice; it was likely just a copy/paste mistake.

Go ahead and rectify that.

sys/dev/virtio/pci/virtio_pci.c

@@ -408,7 +408,7 @@ vtpci_read_ivar(device_t dev, device_t child, int index, uintptr_t *result)
 		*result = pci_get_device(dev);
 		break;
 	case VIRTIO_IVAR_SUBVENDOR:
-		*result = pci_get_subdevice(dev);
+		*result = pci_get_subvendor(dev);
 		break;
 	default:
 		return (ENOENT);