net: 3c509: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning
by explicitly adding a break statement instead of just letting the code
fall through to the next case.

Link: https://github.com/KSPP/linux/issues/115
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/3com/3c509.c

@@ -1051,6 +1051,7 @@ el3_netdev_get_ecmd(struct net_device *dev, struct ethtool_link_ksettings *cmd)
 		break;
 	case 3:
 		cmd->base.port = PORT_BNC;
+		break;
 	default:
 		break;
 	}