virtio-net: mark expected switch fall-throughs

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Addresses-Coverity-ID: 1402059 ("Missing break in switch")
Addresses-Coverity-ID: 1402060 ("Missing break in switch")
Addresses-Coverity-ID: 1402061 ("Missing break in switch")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>