virtio_net: Fix error code in probe()

Set a negative error code intead of returning success if the MTU has
been changed to something invalid.

Fixes: fe36cbe0

 ("virtio_net: clear MTU when out of range")
Reported-by: Robert Buhren <robert.buhren@sect.tu-berlin.de>
Reported-by: Felicitas Hetzelt <file@sect.tu-berlin.de>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/X8pGVJSeeCdII1Ys@mwanda


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>