net: tulip_core: mark expected switch fall-through

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

Addresses-Coverity-ID: 114782 ("Missing break in switch")
Signed-off-by: NGustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/dec/tulip/tulip_core.c

@@ -923,6 +923,7 @@ static int private_ioctl (struct net_device *dev, struct ifreq *rq, int cmd)
 			data->phy_id = 1;
 		else
 			return -ENODEV;
+		/* Fall through */
 
 	case SIOCGMIIREG:		/* Read MII PHY register. */
 		if (data->phy_id == 32 && (tp->flags & HAS_NWAY)) {