virtio: fix boot device hunting in qemu with virtio

Commit 70101c3217 ("virtio: mmio: Return error codes on probe
failures") returns -ENODEV where it would return 0 before. That path is
apparently hit in qemu and breaks boot device discovery
(virtio_bootdev_hunt() expects only ENOENT). So return -ENOENT in
that path instead.
The remaining two error returns in the function are untouched as I
don't know where they play a role.

Signed-off-by: Ludwig Nussel <ludwig.nussel@siemens.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Kuan-Wei Chiu <visitorckw@gmail.com>

drivers/virtio/virtio_mmio.c

@@ -372,7 +372,7 @@ static int virtio_mmio_probe(struct udevice *udev)
 		 * virtio-mmio device with an ID 0 is a (dummy) placeholder
 		 * with no function. End probing now with no error reported.
 		 */
-		return -ENODEV;
+		return -ENOENT;
 	}
 	uc_priv->vendor = readl(priv->base + VIRTIO_MMIO_VENDOR_ID);