[SPARK-20316][SQL] Val and Var should strictly follow the Scala syntax

## What changes were proposed in this pull request?

val and var should strictly follow the Scala syntax

## How was this patch tested?

manual test and exisiting test cases

Author: ouyangxiaochen <ou.yangxiaochen@zte.com.cn>

Closes #17628 from ouyangxiaochen/spark-413.

sql/hive-thriftserver/src/main/scala/org/apache/spark/sql/hive/thriftserver/SparkSQLCLIDriver.scala

@@ -47,8 +47,8 @@ import org.apache.spark.util.ShutdownHookManager
  * has dropped its support.
  */
 private[hive] object SparkSQLCLIDriver extends Logging {
-  private var prompt = "spark-sql"
-  private var continuedPrompt = "".padTo(prompt.length, ' ')
+  private val prompt = "spark-sql"
+  private val continuedPrompt = "".padTo(prompt.length, ' ')
   private var transport: TSocket = _
 
   installSignalHandler()