[SPARK-9657] Fix return type of getMaxPatternLength

mengxr

Author: Feynman Liang <fliang@databricks.com>

Closes #7974 from feynmanliang/SPARK-9657 and squashes the following commits:

7ca533f [Feynman Liang] Fix return type of getMaxPatternLength

mllib/src/main/scala/org/apache/spark/mllib/fpm/PrefixSpan.scala

@@ -82,7 +82,7 @@ class PrefixSpan private (
   /**
    * Gets the maximal pattern length (i.e. the length of the longest sequential pattern to consider.
    */
-  def getMaxPatternLength: Double = maxPatternLength
+  def getMaxPatternLength: Int = maxPatternLength
 
   /**
    * Sets maximal pattern length (default: `10`).