Fix max voltage in ssdrivetrain

The max voltage was getting set to a boolean,
and it was true so the max voltage was 1.

Change-Id: I1cae3783f4a1aea9d64118e041a2825d18861ca8
diff --git a/frc971/control_loops/drivetrain/ssdrivetrain.cc b/frc971/control_loops/drivetrain/ssdrivetrain.cc
index cd29e39..12a6bcc 100644
--- a/frc971/control_loops/drivetrain/ssdrivetrain.cc
+++ b/frc971/control_loops/drivetrain/ssdrivetrain.cc
@@ -148,7 +148,7 @@
   if (!goal->has_max_ss_voltage()) {
     max_voltage_ = kMaxVoltage;
   } else {
-    max_voltage_ = goal->has_max_ss_voltage();
+    max_voltage_ = goal->max_ss_voltage();
   }
 
   use_profile_ = !kf_->controller().Kff().isZero(0) &&