Added missing call to m_ds.waitForData()

It was accidentally deleted in a previous commit[1] and none of our
tests caught it. As a result iterative robot loops run too fast (using
extra CPU).

[1] https://usfirst.collab.net/gerrit/gitweb?p=wpilibj.git;a=blobdiff;f=WPILibJ/src/main/java/edu/wpi/first/wpilibj/IterativeRobot.java;h=0e936de8ec86a76e0a63ddbfc65ec2b88bc71725;hp=d4c5d38a14e29c926389ba1ae84fcbd09ee51f19;hb=HEAD;hpb=726ac3a3d272310b367b25f3c7e15ba6d07b957d;tflink=projects.wpilib/scm.WPILibJ

Change-Id: I663f8619406f26b7fa6fc12bce4444657b829d9f

wpilibj/wpilibJavaDevices/src/main/java/edu/wpi/first/wpilibj/IterativeRobot.java

@@ -151,6 +151,7 @@ public class IterativeRobot extends RobotBase {
                     didTeleopPeriodic = true;
                 }
             }
+            m_ds.waitForData();
         }
     }