Fix LED PWM frequency on button board

The LEDs displaying the value of the analog inputs were switching a bit
faster than intended.

Change-Id: Ibdfafd947efb049aa2f2ed948dd9b166f9efd1f6
diff --git a/motors/button_board.cc b/motors/button_board.cc
index 2e3e2c1..7e4a43c 100644
--- a/motors/button_board.cc
+++ b/motors/button_board.cc
@@ -174,7 +174,7 @@
   }
   ftm->SC = FTM_SC_CPWMS /* Center-aligned PWM */ |
             FTM_SC_CLKS(1) /* Use the system clock */ |
-            FTM_SC_PS(60) /* Prescaler */;
+            FTM_SC_PS(6) /* Prescaler=64 */;
 
   ftm->MODE &= ~FTM_MODE_WPDIS;
 }